Lozlee, the best way to kill your cars aircon system is to let Freddy Fridge get his fingers on it. The substance that is in the aircon system is a refrigerant, not a "gas", and there are literally hundreds of different refrigerants, so how would you know what he charged your system with? Chances are for 20 quid it would be anything that he happened to have in the van at that time. Cars are charged by weight, rarely by pressure and although it is possible to do this is beyond the capabillities of many. I note that there was no mention of pumping the system down or use of a vacuum pump, air in the refrigerant system will result in at best poor cooling, at worst, compressor failure. The good news is that to replace your compressor (You must also change the drier and expansion valve and flush the entire system out) on something like an E39 would only set you back around £700, which really is Ford money.
Get your a/c system serviced every 2-3 years, clean out the crap that collects behind the cooling fan and make sure that the electric cooling fan does work, they are very poor quality and are vital to your aircon systems long life and correct function.
Price for a recharge in the UK? Expect to pay between £50 and £70 plus VAT