Thanks Colin, very detailed reply as would be expected from a 5th Dan. I dont need to measure mine now, I did think it was a norrow one, honest.
My system seems to work fine but I rarely get stuck in traffic in hot weather. The system has been flushed through, heater made to work also (secondry cooling fan if needed 😁) plus the fan has been replaced by the 7 blade plastic version, which I believe was the 'export option for hot countries' such as Normandy 😅
Having the filler on the side does help filling and looking at the level/state of coolant.