do i need replacement part or new boiler?

i have a vaillant combi bolier ref no vuw242 I have had this for about 12 years and now have a leak the water pressure has not dropped but quite a lot of water is leaking from the system. I have had a plumber around who thought this could be repaired by putting some central heating solution through the radiators but this has not worked. I dont want to be ripped off by calling around another engineer and wonder what part needs to be replaced if applicable or do I now need a new boiler any idea what part needs replacing or where to look