Re: Why will my air con not work under 60MPH?
The DIY cannisters have been outlawed but retailers are allowed to sell "existing stock" indefinitely. When it's gone, though - it's GONE! It might well be worth stocking up before that happens!
As for the OPs troubles. No. Belt slip can't account for that. If it did, the belt would be squealing its head off.
It seems odd that you need to reach a particular speed. Do you mean there isn't any air coming out of the vents below 60 or that the air isn't cold below 60? Obviously, as far as human skin is concerned, as the air flows faster over the skin, it will "feel" colder - even if it isn't.
ASSUMING the 166 has a conventional aircon compressor (not all cars do), start the engine cold and leave it to idle. Open the bonnet. Turn the climate control to "max cold" or turn the air conditioning to "on". You should hear a "tick" under the bonnet from the end of the engine with the belts on as the aircon compressor cuts-in. If you don't, there is something wrong with the system, but clearly they are supposed to work standing still - never mind at 60!
All aircon systems will gradually loose their refrigerant - even with no actual "leaks", over a period of time. Most seem to go for 5 years or so between re-gasses. The correct procedure for regassing them is to put a vacuum pump on the system and check that it holds a vacuum for a set amount of time before the new refrigerant is added. This checks for leaks (and gets all the moisture out of the system). The place you take it to ought to refuse to re-gas it if they find a leak, until the leak is fixed, but plenty of cars have just lost their gas "naturally".