You need a fitting called a lead-loc, it is a compression fitting designed solely for this purpose. They come in various sizes related to the lead weight in lb/ft i.e. 6lb,7lb etc, etc.If at all possible cut out a sample of the lead and take it with you to the plumbers merchants. Don't even think about q&b.
DEFINETLY NOT FOR USE ON GAS!! Only way out of this is to consult a registered gas installer for advice/remedy ! I strongly suggest it should be re-piped by a COGI registered gas fitter.