T Blake wrote:

We dont know where to get a large sheet of plastic from here in North West
London, so we are going to stick together 'cut-open' clear plastic bags.

I think you'd be better off finding a source of polythene sheet, rather
than gluing bags together, B&Q or Wickes sell various gauges. The
lightest dust sheet probably not strong enough and the heaviest (3mm
thick) probably overkill for a cloche ...