Drpping overflow?

I have an immersion heater with a smaller feeder tank just above it but all part of same unit. I think It is the immersion that is causing overflow dripping as when I turn on hot water for a while it stops until its refilled. It would appear that water is trickling in to the feeder even though should stop. Trouble is I cannot see as there is no access cupboard. Is this likely to be a washer problem or will the whole ballcock system need replacing. (Will need to come out of cupborad for anyone to do this anyway, beats me why it was put in without proper access).
thanks