I've replaced the seals on the compressors on both my P38's and have recently done 2 for friends as well. Experience tells me it is not worth sodding about with the 'seal kit' with just the ptfe(?) seal - best to buy the kit with the whole arm and the pot or cylinder as well. 2 of those I have done have had quite badly scored pots so you are only fixing half the job with just the seal. In addition, the seal is a complete barsteward to get on and you'll probably bust it anyway. From memory the guys in Sheppey, Kent, Island 4x4 are about the cheapest for the kit.
The first time you do one, it'll take you about an hour from start to finish - including giving the little reed valves inside the compressor a bit of a clean up too. Careful with their 'O' rings unless you have bought replacements for those too - I've not had to replace those yet on the ones I've done and all the vehicles done have been in the 100 - 140,000 mile range.
Good luck!
Best
Simon