Trevor
Everyone I have contacted has for sale some "Heat Resistant Tape". However nobody is prepared to quote an upper limit of degrees to which tape can with stand the enclosed environment of an engine bay.
My son in law has today solved the problem. He is a=the Sales Manager at a main agent and their technicians have a silicone gel that they recommend and use for this exact job. My S-Type is with them this week having new sills fitted and they are going to do the under bonnet job with the insulation whilst the car is with them.
Family connections often prove to be the most productive, and economic as well.
Best Wishes and Regards, John