Firstly, ask your aircon man in Chertsey if he has, or can get the valve cores-failing that, try contacting Polar Aircon in Manchester (They have website). The expansion valve is rectangular approx. 3" long by 1" square , follow the aircon steel pipe around the bulkhead (Under the bonnet) and the pipe goes into the expansion valve which is about 18" from the l.h. wing.
Do your blower fans blow fast if you select manual on your HEVAC and turn the control knob up-the "Bar graph" should increase as you turn it.
If the book went out when you had it gassed, I would think the Blends are okay-are you having any problems with either temperature control to individual sides or air distribution to footwell, screen etc.? i.e. is one side hot and the other cold and uncontrollable-remember you can only have a max.difference of around 6deg between sides.
The best way to find a leak is to obviously put UV dye in the system when gassing up and this will pinpoint it with a UV lamp. If it's a bad leak they will have problems pulling the vaccum down and the vac. should hold. Another method used is to charge the system with nitrogen, which is inert and use a soap solution around exposed joints. Hose crimps are a typical cause on the high pressure side and the condenser (In front of the cooling rad) is renowned for leaking, especially in the corners -top l.h. is the favourite, as the tube wall thickness is reduced at the bends. I had the book showing and found the gas had gone due to the condenser leaking. After this was fixed, a couple of months later I noticed dye around one of the hose crimps at the compressor, luckily I was able to get it re crimped. TIP* Only use hose suitable for Aircon as normal Hydraulic hose will break down-contact your local Pirtek for help or most aircon specialists stock it and the crimping gear.
Aircon compressors are generally reliable, with the clutch being the first thing to give up the ghost.
Come back if you want more info.,if I can offer any more advice I will but don't give up.
