I don't suppose anyone knows from experience roughly how much refrigerant is left when it first stops cooling, do they? Not really vital information, though; so long as it lasts a week, the immediate problem is solved. I'll have it diagnosed by a pro with the proper kit in due course, and if anything needs replacing, get it done then.
I'm not sure I understand what you mean but I'll try to explain how it works so that you can better understand the system and come to your own conclusion. If you know this already, then please ignore this post.
Refrigerant is a gas that is stored in its liquid form in the A/C system as it's under pressure. If the system has a leak, you will lose gas. How much is dependent on the leak.
Some leaks will evacuate the system to empty, others will only appear above a certain pressure.
The (very) basic function of the a/c system is that there is a compressor that pumps freon (the refrigerant) around in a circular motion. On on side there is an evaporator that sits under your dash with a blower fan behind it. The other side has a condenser at the very front of your car.
As the freon circulates, it hits the evaporator in its liquid state. The fan blowing air though it will turn hot air into cold air that comes out of the vents.
The same heat transfer heats the liquid freon to a gas state. This hot gas will be cycled through the condenser to cool it down. So on and so forth.
It's a sealed system and the freon will never wear out. The only thing that can happen is that the system leaks. If this happens, you can 'top up' the system by adding freon to bring the system to approximately the right pressure. You'll know how much you added by weighing the jug of gas before and after the charge.
The correct way, as your a/c tech suggested is to evacuate all the freon using a recovery machine that sucks the gas into a container and then charge the system with the correct amount by weight. There is usually a red sticker under the hood that tells you exactly how much freon the system should have.
If your system is completely empty, the system will need to be evacuated (where a vacuum is applied) in order to suck all the moister out, otherwise it will never function correctly and the moisture will cause internals to rust.
R134a is a very efficient at cooling. In the UK, I would expect that it would function very well under normal ambient temps even if it had half the charge.
My only tip is that you should always run your ac and avoid using the EC mode. The freon also carries the oil around the system and without the circulation, seals might dry up resulting in a leak that would not normally be there.
As far as detecting leaks, there are various methods to figure this out including sniffers that can detect gas, UV dye that can be added to the system or good old soapy water which you can spray and watch for bubbles. A good A/C tech will be able to track it down and as long as it's not behind the dash, it can be fixed at relative low cost.
Good luck.
