That depends, you most likely have 7lb 1/2" lead for that job but I have come across 6 lb 1/2". The 1/2" is the internal diameter the weight tells you the thickness of lead it's made of.

If you go to a plumbers merchant and ask for a 7lb conex leadlock on it are given the circumferance, go home and check your pipe with a piece of string.

If you end up with a lead lock that is too tight to get on file the lead, if you end up with a lead lock that is too slack, open out the pipe.

The lead lock gives you conversion to 15mm compression fitting.

An easier option is to go to Plumb Centre who sell a plastic compression fitting that is range rated. Once again go and see what they've got and measure the diameter of your pipe, to see which you need. This is the more expensive route. Word of warning the instructions say hand tighten then 1 turn or something like that. That's ********, tighten it with grips very hard or it will leak!