I have noticed when central heating is off boiler pressure drops from 80% to 25% in 2 days. when ch is on takes 3-weeks.
How can I know if there is any leaks as visible pipe work and radiators look ok.

Patel, March 2008

Not 100% sure but if no leaks on system then probably an internal leak in your Domestic Hot Water (secondary) heat exchanger. Allowing central heating pressure to pass into DHW system.
Solution - Replace secondary heat exchanger. Old black "plastic" exchangers are known for this and are obsolete. Replace with new metal exchanger.

Colcreate, April 2008