Baxi Maxflo Combi WM losing water through drain

My daughter has recently bought a flat with a Baxi combi which frequently requires re-pressurising. She has noticed that water is dripping from the external drain when the CH is on which explains why re-pressurising is necessary. It is also reaching 3 bar when this happens which I understand means the pressure relief valve is opening.

Could you please suggest what may be causing this high pressure. Could the pump be at too high a pressure level? Thank you for your help.

Not the pump.

Expansion vessel not charged. Either pump it up to recharge it or the diaphram is split and it needs replacing.

Or fit an external one on the return pipework if you have space

Thank you powell30. Is recharging the expansion tank something I can attempt? If so could you explain how it's done. Thank you