Are you sure that the fan belt is too loose? Overheating can also be caused by so many other things. When your bus is at the mechanic he should be able to determine the reason - radiator blocked? stuck thermostats? unsealed air gaps? slipping water pump impeller? constricted exhaust?
PS - I had the pleasure of visiting your namesake, the RN's first nuclear sub, when it visited Rotterdam in the 1960s. Very palatial compared to the usual scrotty smelly diesel subs.
I'm not sure no, but I'm tracking down the most probable root causes and knocking them down, one by one if neccessary. There are no shutters (even though the book says there should be) , the radiators although not new, look in very good condition. A thermostat is possible- which is another one that needs to be tackled. When I drive it again around the block- I'll see if the radiators get hot to check that. I want to change the water pump anyway- they're about $350-500 but my mechanic says that if there's nothing coming out of the weep hole...
I may get a new one anyway, if the engines coming out- it probably wont cost a lot more in terms of labour to get a new water pump fitted. The cooling system holds pressure well.
My favourite Dreadnought was the one from 1906- 1915 from WW1. It didn't see much action but was a formidable ship and show of power for the British Empire. Sea Lord Fisher who comissioned it seemed to me a bit of a nut case though!