Depends which part of the right side is leaking.
The two main items are the diverter valve and micro-switch assembly and directly above that is the cold water expansion vessel(silver ball with small pipe feeding into it).
Two different sets of water. The valve is part of the heating system and could be causing a loss of pressure on the gauge.
The vessel is part of the cold water mains. The pump is also there with several small supply pipes and an auto-air vent.
Vaillant boilers of this type are well constructed appliances and most parts are available. Many of the major components can be repaired with service kits although i wouldn't recommend d.i.y.. there are a couple of safety measure's built into the hydraulic operation which could cause complications if you get it wrong.
T.h.s
May 2007