The air in the car has been getting warmer recently, so I took it into Kwik Fit for an air con recharge (this worked very well about 3 years ago). They checked the vacuum (all ok) and refilled it, but then said that they would not charge me because they had failed to make it 5C cooler and to check the compressor.

- I have visually checked the compressor and the front section nearest the radiator is stationary with the air con switch off but rotates with the switch on (so the wiring and clutch appear to be ok).
- The system holds a vacuum and there is a fresh charge of oil and refrigerant in the system.
- A second blower fan appears to kick in when the air con is turned on
- Both air con fuses in the front funbox are intact
- Using my IIDTool, there are no error messages for the air con ECU (HVAC?)

If the compressor clutch is rotating and there are no leaks, what else should I be looking for (hoping that a failed compressor would leak or not rotate...)?

Have you checked the outlet air temperature at one of the face vents with the ac on. Give it time for the system to cool
down. You should get temperatures of about 4C at the vent.

Sadly yes - it was no cooler than ambient (15C ish) but I'm used to the 4C chill!

That does rather sound as if the compressor isn't compressing.... if the centre of the compressor is turning, then you should be getting a drop in the temp..... sounds like you might need to seek a specialists help.... although I did come across a similar thing, and that turned out to be an over charged system.... it overheated almost instantly and caused the electric fan in front of the rads to be on continuously, and the ally pipe at the top left corner of the rad got stupidly hot  Yesterday I couldn't spell Engineer... Today I are one!
Inventor of the 'Guide-o-Matic automatic wheel alignment tool'
Former long term L322 owner, Up/Down graded to a Classic Tractor!

I'm confused (admittedly not very difficult...).

I found a gauge which I had used previously to top up another cars air con.

With the engine off, the low pressure port is at 35psi (which suggests something is in the system under pressure) but down to zero psi with the air con running at max (so the compressor seems to be doing something!).

Does anyone know if those values seem roughly right?

The aluminium pipe between the high pressure port and the condenser does seem to become quite hot (50C) and the low pressure pipe out of the condenser gets down to around 10C.

Fixed ...

For anyone also having similar problems on a UK 2006 TD6 (your RR may be similar, but I don't know!):
The high pressure port is a 16mm R134a snap on and is physically the highest one on the condenser (by the radiator)
The low pressure port is a 13mm R134a snap on and is physically the lowest one on the condenser (a coupe of inches below the high pressure port)

I used a gauge on the low pressure port and monitored the high pressure using my IID tool.

When the air con wasn't working effectively the readings were:
Engine off: low pressure 35psi (shows that there is something in the system)
Engine & compressor running: low pressure 0psi, high pressure 13Bar (both seem low)

Digging around the Internet (and many thanks to miggit for additional data), the low pressure needed to be around 25psi and the high pressure typically 15-17Bar with the engine & compressor running.

So I purchased an 18oz can of R134a (the under bonnet sticker says 18oz) and connected it to my existing gauge and the low pressure (LP) port:
- Start engine and put air con to MAX for 3 minutes - initially readings were LP 0psi, HP 13Bar, ambient temp 10C
- I slowly filled in 30s bursts (making sure to keep the can upright in order to add gas not liquid) and carefully waited for a pressure change after each fill. Ending up with LP 24psi, HP 15-17.2Bar and nice cold 4.6C air out of the vents (and 35psi LP with the engine off).

I can only guess that the previous air con service had perhaps not added quite the correct amount of refrigerant - I guess that I have used 12oz to top things up.

That'll teach you to trust a tyre fitter with your Air Con

Glad it's all sorted, just in time for the snow !  Yesterday I couldn't spell Engineer... Today I are one!
Inventor of the 'Guide-o-Matic automatic wheel alignment tool'
Former long term L322 owner, Up/Down graded to a Classic Tractor!

guilty as charged (A different branch 3 years ago were very good and I had hoped that the automated machines would remove most of the operator error...) ! Thanks for your help