On 20/05/2018 18:12, Andrew wrote:
Has anyone done this ?.

Local garden centre sells it, so easier to purchase but they don't
sell the adhesive that you need to make lap joints.

I would also need something to finish the top of the upstand on
three sides while the gutter end folds under somehow.

I used an offcut on the top of my 3x2m coal bunker and it has held up to
UV and sunshine ever since but I suspect it is close to the maximum size
without getting into thermal expansion issues. It has lasted somewhat
better than the classic roof covering it replaced which didn't really
get on with the flexure associated with opening and closing the lid.

I'd be wary of using pond liner on a roof since it isn't engineered to
stand up to that duty. ISTR there are variants which are on sale.

--
Regards,
Martin Brown