Going back to the other day re: my very leaky flat roof.. Thanks for all the suggestions. I popped into Wicks this afternoon for something to effect a very emergency repair with due to severe weather warnings...
Came back with 2 cartridges of their "felt lap sealant" type stuff. Squeezed it in/under/along the rather open lap seams in a bathroom sealant sort of way and gently squished the top layer down a little with my shoe.
So far, after some rather large rain it's holding up. No drips yet so fingers crossed it'll last out the winter or stop the worst coming in at least.
£12 well spent for the moment at least...Pete@