Thanks for all the replies... the melting the pipe option was soon
discarded and I went immedialtey to plan B - lead line (like lead lok,
apparently). Had to have two goes to make it watertight - the first
attemp the outside of the pipe was not perfectly round and smooth...
after much medium-then-fine sanding the second attempt sealed it
fine.... fingers crossed itstays that way! I'll be ripping out the
lead at some point, so only needs to last until then!

Thanks again for all the help!

Simon