Your overheating problems sound the same as what I had on my D plater many years ago.

Try a new header tank cap(it may have failed) sort out a new thermostat, if possible go for a lower temp one. Flush the coolant system taking hoses of from the heater matrix on the bulkhead and put a hose to it. Same for the radiator too. For the sake of it i'd be looking at a new fan switch and fitting a new modern fan or 2. maybe a fan over-ride switch fitted on the dash somewhere.