T Blake wrote

We want to make a Cloche with hoops of bent-over plastic water pipe,
covered in Plastic sheeting; to end up like a minature Polytunnel for the
allotment.

We dont know where to get a large sheet of plastic from here in North West
London,

You must be able to get it there from those
that supply those that do greenhouses with it.

so we are going to stick together 'cut-open' clear plastic bags.

That's mad.

What would be the simplest way to stick the bags together,

There isnt any way that works as well as starting with the right stuff.

and if glue is the answer; what is the best type of glue to use please?

No glue works very well.

Bearing in mind we would probably have to use lots of it and it would have
to be one hundred percent waterproof.

Best to start with the right stuff instead.