I met someone at a show a couple of years ago who was selling recycled plastic stuff. He had some panels that were 2m x 1m x 10mm thick that he was confidently expecting to sell at £500 each. I was looking to install a load of showers at the time and could have used these instead of ceramic tiles - but not at that sort of price!

Needless to say I didn't keep his details.

There is a company that makes decking and fencing profiles out of wood and recycled plastic composite that would be ideal to use for a picnic table. Don't know how expensive it is though: http://www.ecodek.co.uk

Fix together with stainless steel screws or nuts/bolts/washers.

ted