Happen to have this handy:
The red bit on the left, probably. Tap water.
If its the purplish bit, tough, you'll have to buy a whole diverter unless you have a box full of identical bits from other boilers' service kits like we have!
I know it's an old thread, but I have a Baxi 105HE, and it seems my boiler is dripping from the bottom right bolt on the blue bit. Would anyone be able to advise? Would it be done by using the service kit mentioned in this thread?