follow on info from combi boiler low pressure?

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We have just had an additional radiator fitted to our central heating system. Each day the pressure has dropped, when topped up the system works fine. We have checked for leaks round the new radiator, nothing found. The system has worked fine until now,
ANY ADVICE WOULD BE HELPFUL

The radiator was fitted by a corgi plumber who is away until wednesday evening so I do not know off hand where the system was bled.

The one thing that I have noticed is that when the system is filled to its recommended pressure and the heating is put on there is no drop in pressure. Once the heating is knocked off the pressure gradually drops.
There was no problem prior to the new additional radiator being fitted last tuesday.