Do you find that your boiler fails to fire because the pressure is low? Have your radiators been bled recently? it could be that pressure was lost during that process!. If not, this pressure drop indicates that there is a leak somewhere within the system. If you find that you are re-pressurising your boiler on a regular basis, this would suggest that the leak is significant. Check for damp beneath your boiler, now check all your radiators including heated towel rails in bathrooms. It could be as simple as needing to tighten valves. If in doubt, give Farringdon Plumber London a call.