Overflow query

The overflow pipe from the tank in the loft has been running intermitently. I noticed today that it started about an hour before the heating was due to come on - it was like a slowly running tap. Within approx an hour of the heating coming on, it has reduced to a slow drip. This happened for a few days previously, then nothing for 4 or 5 days, but today - it is back.
I did have the "valve" in the airing cupboard replaced (the thing where the switch moves when the heating comes on). I know that the chap who did it had to drain some water from the tank up in the loft. Could it be related to something he did? (this was about 6 weeks ago + the overflow problem is more recent).
Sorry that I can't give more info - I am a typical female novice when it comes to DIY (have never even been up in the loft!)

Which tank is overflowing?

Does it feed the cylinder or heating?

You mean there is more than one tank up there?!
How will I know which one it's coming from? It's coming out of the overflow pipe onto the patio (but not the pipe which is connected to the bathroom).

You will have to investigate and post back as I cant see from where I am