[Electrickery]

I'm also interested in this, from a gas boiler POV. What are the optimum conditions for 'condensing' to take place, from which an efficiency benefit is derived and how to determine the quantum of efficiency?

I gather gas boilers manage this better as they modulate the temperatures down and vary the pump speed to spend more time in condensing mode.

 

[Electrickery]

I'm assuming you have an opentherm thermostat or an outside temp sensing device? If so it won't matter what temperature you set it to as the opentherm will take over and modulate the radiator temp down to suit the set thermostat setting regardless of your max setting on your boiler.

Equally the hot water in my eyes shouldn't be set any higher than you can just about manage to hold your hand under. Having the hot water temp at 75 is far too hot in my opinion.

No, open therm isn't really a "thing" on oil boilers, and it has Hive which doesn't have open therm but does I believe have some learning ability (but that is really just either for preventing overshoot or having a "ready by" time for heating. But it can't control the water temperature or pump speed directly.
As for hot water there is a cylinder stat obviously so that is set to around 55/60, but if you don't have the boiler flow at at least 65/70 it will simply take an age to heat the cylinder.

 

[Electrickery]

I have a Grant oil boiler. It's ostensibly a condensing boiler. Erm it as far as I can tell more or less never condenses. I have never seen anything come out the condensate pipe, though I've never watched it or attempted to collect condensate. At its minimum setting still great plumes of steam.
My conclusion is the optimal setting is off. Which is where mine is 99% of the time now I have air con and I use Go with the immersion for hot water.

Yes mine can give off a lot of steam, though it does become more noticeable in certain weather conditions, and at first start up from cold it is quite minimal, like a small vapour trail.
You can fit a plume management kit to it, I wonder if that effectively creates more condensate, as presumably that will condense more in the flue and run back into the boiler, as to whether there is any additional useful heat doing that though.
I presume it is still way more efficient than older style boilers as the flue temperature is much cooler, you can hold your hand in front of the flue without discomfort whilst it's at full flow

 

[Electrickery]

The condensate drain pipe on my boiler is however always cold. Hence I suspect no condensate.

It would only be a trickle anyway, and wouldn't be noticeably hot.
They have in the past caused many boiler breakdowns (I recall a really cold snap in the early 2000s where British gas were inundated with fallouts for this very issue) when they freeze up in really cold weather, so they use bigger pipes or route them mostly inside these days.

 

[Electrickery]

I used to run my Worcester Bosch boiler on its frost setting. Using an infrared thermometer it turned out that the cut off point was ~40 degrees and cut in ~36 degrees. At that temperature is must have been condensing all the time. In the damp misty weather it would still plume.
Having established I could heat the house with the boiler at that temperature I moved to a heat pump. That was installed in September. I just checked yesterday, it has produced 8600 kWh of heat for 2570 kWh of electricity. That was disappointing I was hoping that I would get a COP nearer to 4 not 3. But I have used it overnight, load shifting the electricity demand consequently it has been running in colder temperatures to heat hot water. And SWMBO enjoys bathing in water drawn at over 45 degrees. So by the time the water has cooled in the hwc from the night before it needs to be heated up to 55 degrees. Cost for the winter so far has been ~ £365, we were away in December for some weeks, but I believe the weather was relatively mild, I operated the system when the outside temperature got into negative figures anyway.
For those whose radiators are undersized, I would look to swap them for double type 21 or 22. It is a simple diy job, close the valve and lockshield off tight undo the compression fitting on the valve, catching the water, and drop in one of the same width. Some radiator widths are no longer supplied, but you can get nice shiny chrome extension tubes to take up the gap if you fit a size smaller, e.g. 1300 cm wide is now rare but 1200 common and cheaper being oft demanded. If you need, use a bit of tpfe paste around the compression joints for added lubrication and sealing properties. Radiators shot up in price last year, many of them available from the 2 well known trade/anyone counters, are made in Turkey. Rampant inflation there is likely to encourage a reduction in price. In fact I noticed that this appears to be happening already on the common sizes: 600x1000 type 21/22 and 600x600 doubles.
Using my heat pump at lower temperatures and leaving it running almost constantly has given us a very comfortable house, not over warm, mostly around 19/20, a little warmer where we sit in the evening.

Yes I've noticed radiators have increased in price a lot. Most downstairs rads are double, upstairs all single panels. I probably need to replace one of the singles with a double anyway, it's for one of the bedrooms. The problem is that the thermostat is in the lounge which has one huge double and a single convector, so the lounge gets up to temp very quickly, and doesn't give some of the upstairs rooms time to warm up. The lounge is south facing too so is usually a couple of degrees higher than the bedrooms so has a huge head start.
Short of fitting smart trvs all 11 rads in the house (plus 2 rowel rails?!) all I can do is hugely restrict the flow to the lounge so they heat up very slowly giving the upstairs more time to heat up. Or fit oversize rads upstairs with normal trvs.
Bizarrely I need a 900 wide rad for the particularly cold bedroom. A 600mm high type 21 rad is around £100, yet a 700mm high one is only £68. Type 22s are slightly cheaper than 21s too, so the pricing seems all over the place! But type 22s are more troublesome as the pipe centres are quite a lot further out than singles, type 21s you usually have enough movement to work with.

 

[Electrickery]

The drain on mine is 32mm waste pipe straight into the ground.
It produces masses of cool vapour this time of year.

Yes I think 32mm is std now, but it should go into a soil drain or neutralising trap (limestone?) rather than straight into the ground.
I assume you mean steam vapour from the flue and not from the condensate pipe?
From cold start mine gives off very little steam from the flue, and a good trickle of condensate, but once the return temps hit around 30C plus, very little condensate and lots of pluming.
Basically if you are heating the DHW the boiler will not be condensing at all, so I tend to time the hot water to come on about 30mins after the heating starts