Oddly enough it seems to've sorted itself out now

I am doing a few mods to it but the only thing I've done so far that I can think might've been affecting the heat is that I've done away with the stock air filter box and filter pipe altogether and did a complete new filter/pipe setup

Didn't bother looking but maybe the air filter was overdue a change by a few years or something, the car had barely been driven since 2007 so I also doubt anything much was done to it in terms of maintenance/servicing during that time either

I suppose it's conceivable that if it wasn't sucking enough cold air in it could cause the car to run hot, there was even a pile of leaves sitting on top of the intercooler when I first popped the bonnet lol