Every couple of months have to top it up. Rads not leaking as far as I can see. Does this mean there is a leak in the pipes somewhere? I hope not as there is laminate floor laid and wouldn't know where to begin with that?
I am on maintenance contract so what might the boiler man do? Will he want the floor up to test the pipes?
Months then I wouldn't worry too much. Could be a slightly loose radiator valve with an unnoticeable leak. Are there any signs of a leak under your boards? We had a damp patch on the ceiling below our leak, so it was quite obvious where to pull up the boards!