The air leaves the engine bay mainly via the gearbox tunnel, but that only works well enough when you are moving at speed.Hot air rises - and where is the fuel pump? Up high at the back..... A couple of grilles in the trailing edge of the bonnet could work wonders both for fuel vapourisation and engine cooling.
The Minor coped fine in standard form in 40 degree ambient conditions when being thrashed through desert sand, so the root cause is related to lack of air flow (allowing the area to get too hot).
Hence this happens only in traffic jams.
Holes in the bonnet for cooling (this isn't a new idea) have a look at the Himalayan minor ;-) Their mods were done on advice from the knowledgable guys at BMC.
