You have UFH .. so already a big plus for ASHP.
What's your heat loss? 15kWh would be used in 3hrs on our 12kW Vaillant heat pump on a very cold (sub zero) day.

The most important things for efficiency is flow temperature. The lower you can go the better .. which means you need large radiators (or UFH) ! Factor in replacing some of those rads .. you can do it over time and don't have to do a big bang.

Our SCOP this year is 5.2 on the aerotherm+ , with Octopus Tracker averaging 18p/kWh , it's working well

Got for it !

 

How much gas did your Vaillant use Nov,Dec,Jan,Feb ?(total)
How many pax living at home?

 

You're starting from a better position than most, including my journey with Octopus and ASHP.

A lot will depend on your heat loss calculations and hence whether you may want to consider further investment in additional battery capacity if you are already fairly tight to the 15kW you have already. You don't want to just install battery capacity for the worst few weeks of the year temperature-wise, but if you would be reverting to grid import for the majority of winter months then that is a different consideration.

You don't state whether you have microbore on the upstairs radiators, because if this is under 10mm then it needs to be swapped out. Even at 10mm, the Octopus surveyor may pick up on this.
I assume you have good insulation, good glazing and doors, and an up-to-date EPC rating, as all are effective prereqs before considering this in the first place.

As @Cheshirecat said above, you need to know current gas usage and household profile, so that you can do some maths later on once you get the fully detailed survey results.

I would put down the £500 deposit and get the survey scheduled as that alone can take months to get a slot, whether you go with Octopus or British Gas (the two big suppliers doing this unless you want to go with someone more local). You are in no way obligated to proceed after that, and you can get the deposit back if it turns out it doesn't work out for you. Remember that the grant is a fixed budget each year so it will get increasingly more difficult to obtain as installations of ASHPs pick up.
Certainly, a standard water-based heat pump rather than air is going to be the right technology for your house given the current heating setup.

I would go ahead and move to an induction hob instead of gas hob because it's something you'll have to do independently and it's one less thing to consider later on.

I am just on 11 months with my Daikin 6kW ASHP, and for heating my average COP is 3.8 with probably >95% of my heating via the IOG 7.5p/kWh off-peak rate directly or via the same added into my 13kW of batteries. Not a brilliant COP value, but not horrible either.

 

How old is the boiler and Megaflo? Had any reliability issues lately? The Megaflo will need probably need replacing if you get a heat pump.

 

Try Heatgeek as well, in our experience they were able to survey us quicker than Octopus and at £350 for the heat loss survey its good value compared to £500 for Octopus even if you decide to go with a cheaper installer. HeatGeek target low flow temperature which involves replacing radiators and pipework but results in high Scop and cheaper running costs. Octopus might go with a higher flow temp so can use your existing radiators and pipe work but will result in lower Scop and higher running costs but cheaper install. But if you have enough battery and run the heat pump during your cheap tariff it might work out cheaper overall, the choice is yours 🙂

 

Is the Heatgeek survey cost of £350 fully refundable if you decide to not go ahead? Because I believe that is the case with Octopus and you get the full £500 back.

Is there any evidence to back up the statement made that Heatgeek will design to lower flow temp vs Octopus for equal-sized properties because surely they are both just working to the MCS standard? They will each have their methods and standards but to what level that would drive a material difference in calculations and design that isn't more driven by the choice of hardware isn't clear.

Has anyone had both a Heatgeek and Octopus survey done on the same house so that there is an apple vs apple comparison when discounting differences in hardware manufacture?

I'm not saying a Heatgeek installation won't be better than one by Octopus, I'm just trying to get a sense of why and the key areas of difference in either design or survey that drives that.

I can't comment because I just went with Octopus and no other surveys. They did replace all 10 radiators in my house to achieve their design against a 4.9kW Building Heat Loss and 50C Leaving Water Temperature with a Design External Temperature of -1.9C. Their design defined a SCOP of 3.43 for heating, and at 11 months since installed my average shows a COP of 3.8

 

Is the Heatgeek survey cost of £350 fully refundable if you decide to not go ahead?

Absolutely NOT refundable

 

When I checked a few weeks ago, Heat Geek were guaranteeing a SCOP of ~3.8, I think, for every installation. They seem to have backed out of this now, guaranteeing that the installation “will be at least as efficient as quoted on your proposal”.

 

The Heatgeek installers unofficially compete amongst themselves to get the highest SCOP but I'm sure other installers can do a decent job as long as they do a proper heat survey and size radiators, pipework and heatpump accordingly to get the most efficient design.

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Octopus initially offered me a very reasonable quote of £1100, but the order was failing. When I contacted them I discovered they don't offer installations for houses larger than 220 square metres because it requires a heat pump larger than 14kW.

Which means they'd install a heat pump larger than 14kW to replace a 28kW mains gas boiler. That's a long way off a SCOP of 3.8!