Hi Mark+4
Your foam is not quite deep enough, it has to seal to the bonnet under surface so that's where your hot air is still bleeding in to the heater box. check the closed bonnet side by looking through the open bonnet for fit.
Don't remove your bonnet sealing blocks, rather cut a slot into it where the bonnet stay wants to sit and fit the foam around it as you fix it in the stowed position.
Oh! and that's a plus +1 for not messing with an original radiator, buy a cheap syringe and some tubing and suck it out of the expansion/ header tank.
