Hopefully i can help..
When does this overheating usually crop up? underload / sitting in traffic etc..
I see you say you have to top up coolant is this regular thing or just something you spotted first time around since the fault occurred? have you noticed any leaks appearing under the car? or any excess pressure build up in the expansion tank?
As you took 6 hours to come back with info ,the AA gave it in 5 MINUTES
I WOULD LIKE A FULL RE FOUND