how to extend hard top?

[Rigger Mortice]

Seems like the answer is 'yes-if you get a good bit' or 'yes-if you turn it into a fibreglassing exercise'...which I can see being a laugh over a 10'x8' or so area....
Let's hope we can find someone to do it for us (him)

It's an easy job. If you can handle a paint roller you can do that job.

 

[Tranona]

If you're doing that (which I agree I would) then the expense of true marine ply seems a bit unnecessary, versus decent WBP.

Pete

For the area he is talking about, the difference between good WBP and ordinary Robins Elite, assuming 12mm would be about £100.

The problem with large horizontal expanses such as that is the need for joins. Ideally sheets should be scarfed together, but butt joints over beams are normal and are the weakest points (along with exposed edges). With new work it is easy to sheath with one layer of glass cloth right across and over the edges. You could even go down a thickness on ply and use two layers of glass.