Replacement Filling loop isolator Valve?

Hi,

Recently baught a flat and need to re-fill the CH system after bleading the radiators only to find that the valve on the left hand side is broken (the circular turnable blue plastic knob is missing) the one on the right hand side is AOK. Any ideas as to where I can get a replacement one ? (my local guy seams is taking A-G-E-S to "get that part")