As what agile said,it could be a sales trick for you to buy a new boiler.
original Part is oboslete but a kit is available which includes the valve,pipe,lead,seals and other bits.
ebay uk often has them search, "glowworm ultimate gas valve kit tandem"
Had one of those boilers and it was very reliable till man in blue van touched it.
Make sure the valve will cure your fault,these boilers are easy for a registered gas person to repair.
Good luck.