Ta again Geoff,

geoff wrote in message ...
Slightly different principle here. You have an inverted "T" from
aluminium which butts up to the two pieces you are joining. There is
then a mushroom shaped top which slots into the "T" with barbs. You can
buy the stuff in 6 metre lengths, so there is no need to overlap down
the length.

Since I want to drain the rainwater down to the end of the carport, I'd
guess that a 6 metre length would be the way to go (presuming I can get
the twinwall in 6 metre length)?

I bought mine from PolyPlastics in Enfield (which is where
Screwfix buy it from (I think)), but there must be other distributors
oop norf.

Any non-obvious web-site you know of that Google doesn't (I will look,
I haven't yet :-) ?

I'll take some photos over the weekend for you.

Appreciated; thanks.

Mungo