Yes Arthur , this is a perennial problem.
Modern engines are designed to run hot to keep emissions down, so the heat flows back towards the passenger compartment. Great for keeping you warm on winter drives, but not so good in this scorching weather.
There are various methods that folks have tried . One is to use an insulation material under the covers of the transmission tunnel and bulkhead. Another is to make sure that there is nowhere for the hot air to enter the passenger space from the gearbox or handbrake by fully sealing these. Yet another is to use extension clips for the rear mountings, so that the hot air bleeds out from the back of the bonnet.
Finally there is now the option of air conditioning, perhaps a bit more tricky as a retrofit to an older car , but should be possible. The Rover engine in your +4 would almost certainly have driven an a/c compressor in its saloon environment, it may be tricky to site all the components , but where there is a will there may be a way.
