I re-filled mine with a canister from the USA 5 years ago as a temporary measure, it's still going stronghave you or anyone else put one of those halfrauds ac gas canisters in?if the system is not vacuumed properly you get moisture in the system which when it reaches the expansion valve freezes, and blocks it,your scenario sounds very similiar to this
If the viscous fan is working, the electric fans are unlikely to operate in typical UK summer temperatures unless in a queue of traffic with the outside air temperature in the mid to high twenties, they rarely ran on mine until I used them to replace the viscous fan.are your ac fans working? if they are not, then the system will over pressurize and shut down.
Should be just to the left of centre at all times, maybe your viscous fan is on the way out, do the rolled up newspaper test.Regarding the cooling, with high ambient temp, the cooling gauge sits a bit over centre, I use the car for long distance heavy towing across europe, and its always been steady, except on very long hills when it will reach one division over centre in summer, but no loss of coolant, but keep a close eye on it, will keep you posted