Thanks for your comments.
Ignition and carb settings have been checked and rechecked and are all found to be OK. Besides that, all wearing parts of the SU carbs have been renewed not long ago. Air leaks could not be established. Under the hood almost everything is either new or reconditioned.
To solve the vapour lock problem, I will first have a heatshield made as large as possible from stainless steel, or actually one in two parts, of which the second part should be easily unbolted to restore access to the oil filter, the rear of the alternator, or to the exhaust flange nuts, when necessary. Also I will remove a floor pan in the engine room that sits in front of the cross member, which will allow for some more fresh air under the hood. It’s mounted with four screws only, though the IPD sway bar hides the front screws and will have to come off first. I already removed a circular piece from the RH radiator support panel, in fact the mirror of the fresh air opening in the LH support panel.
If all this will not help, then it still may be necessary to improve the fuel hose isolation. I think in that case I will have a stainless steel pipe made that fits the fuel hose like a glove.

If nothing works, then there is a final means with success guaranteed: converting to LPG, which is considerably cheaper in this country, though installation cost set you back initially but can be earned back by driving a lot. With an LPG setup BTW, an injection inlet manifold (see earlier post) would be ideal as it will give the most bhp. I’ll let you know what the solution will be.