Thanks for all that, Tom. A great “How to”.
Unfortunately it was my garage that broke my perfectly-good skylight and THEY replaced it – I had no input!
We used to use a product “Dum-Dum”, which was an inert, rubbery “putty”. I’m pretty sure this is no longer available. A “bedding putty” or “body putty” would probably be what the grey stuff is, but I can’t say for sure, sorry. I think I might be inclined to use one of the Sikaflex products.
Best wishes.
Barry