Latest update :-
Managed to stop the boiler tripping out on overtemp but I don't know why it was doing it. After swapping both the boiler & overtemp thermostats the problem was still the same, so I looked at the radiator in the small front bedroom, this is the 1st radiator in the line from the boiler and makes a 'wooshing' sound each morning as the boiler attempts to heat the hot water up. As this is one of the only radiators with a thermostatic valve which I hadn't removed I turned it all the way up to its max, now the boiler does not trip out but that room gets quite hot, incidentally, when the hot water is on this radiator gets warm, I can't understand why this should happen, as on a Honeywell 'Y' plan system such as mine if the hot water is only on then it should flow straight from the boiler via the pump, through the 3 way valve, into the coil in the cylinder and straight back to the boiler again. I don't think there is any air in the system as i've bled it so many times & no air comes out, also all the radiators heat up quite quickly with none staying cold.
Also the system does have a by-pass in the garage where the boiler is which isn't lagged.
Any ideas anyone as i'm starting to think that the system is plumed incorrectly as I don't think it started its life as a 'Y' plan system I think it first was gravity fed and then modified sometime in the past.