In message , Dee
writes

Thanks Geoff, they are just great instructions. Exactly what I need.

One question please. You say silicone - not sealant?
A quick look today in the DIY store shows rows of 'silicone sealant'.
Costs around £3-5. Are we talking about something else?
Do you know any brand names I can look for?

I mean silicone sealant

Well, I use silicone sealant from RS - mainly because I know that it's
decent quality and will hold 1/3rd of a ton of water

For your requirements, I would expect that regular silicone sealant from
a shed will do, just make sure that it's silicone and not acrylic

--
geoff