[Dave Barr]

Hi Andy, I've done it several times myself on my Q90. It's really easy and for a man with your skills it'll be a cool breeze. Halfords do AC/Pro, and used to do STP, both of these have the oil lubricants.
You attach the kit to the low pressure tap, which is on the drivers side (RHD) and can easily be identified by the valve being on the very thick metal pipe. The high pressure valve is on a thin metal pipe on the passenger side.
One thing I did do was get the valve sealant from Halfords which actually worked for me, and then did the Aircon top-up the next day.
My pressure reading went from 2.8 (compressor not kicking in at all for the heating) to 5.1..and then the heat pump worked fine (it was March I think).
Not had to top it up since. Compressor still sounds like a jet...albeit a Lear jet and not 747 now.

 

[Dave Barr]

Oh, always use R1234YF...you can't mix the gases at all. Yes, it is expensive but all modern cars use this now.

 

[Dave Barr]

Hi Andy, give me a shout if you need any help. I actually did it at first as my heat pump wasn't coming on in winter as the pressure was too low. Below 5c and the PTC heater came on...but higher than that then heat pump refused to activate. The seals seem to wear a bit, or dry out, over age and it was recommended to me to put the sealer in first. It has worked very well. You'll see different people give bar pressures but it's all down to ambient temperature and humidity. I'm in Scotland and when it is 10c I get about 5 bar pressure before I activate the heating or AC...when running it gets up to 11 bar at ~7000 rpm.
Yes, the gas is expensive but better to pay for a full bottle than pay even more just for a "top-up"

 

[Dave Barr]

13 bar when working hard is on the lowish side, I would expect 18-20. If I were you I would consider replacing the refrigerant indeed. And as indicated earlier, do absolutely use the R1234YF type.

While at it I would insist you / they also
1. add dye so if there is a small leak, you can detect it with a UV torch.
2. have the Schrader valves replaced. It's the most common leakage point and they cost next to nothing. If you hear a soft "psht" while unscrewing the blue plastic protection caps on the service ports you can be SURE there is a valve leak, but the reverse is not necessarily true.

Oops...good point about the Schrader valves. Mine had the minor leak noise but I just used my valve tool to tighten it and never had the leaky noise again.

As for the pressures, I just go with the guage...Scottish temperatures don't lend well to pressures over 12 bar when running full blast. Along as my heat pump works that all I care about.

 

[Dave Barr]

have they got the same type of valve as a tyre / tube has in its valve ?

Actually yes, I used a tyre valve tool from Halfords when I was replacing my valves in my old 2008 Prius.
It's that simple... embarrassingly simple. It was another thread that alerted me to it...and I used my very cheap tool to tighten the valve.