If the boiler is fairly new, the 3-port valve in my link may be inside the casing.
There may be 4 large pipes coming out of the boiler, probably from the top, which are (should be) inside plastic foam lagging.
Two of these pipes will go into the cylinder, a flow and a return. After you have run a bath, these pipes will become hot as the boiler pumps heat into the cylinder. The flow will be hotter than the return.
When you put on the central heating, the other two pipes will heat up as it pumps heat to the radiators.
You should be able to detect which is which by observing when they heat up. You might likje to stick labels on then for next time.
I suspect that when the CH is running, heat is leaking into the cylinder, and when the cylinder is heating, heat is probably leaking into the rads.
It will happen even if you leave the HW set to "off" though it may take a long timne to heat the cylinder.
If you turn the CH off at the timer or room stat, and run a bath, feel if the two CH pipes, or any of the radiators start to warm up at the top.
Because the weather is cold now, the VCH will be running a lot, and I suspect that excess heat is leaking into the cylinder.
In this case it is probably the 3-port valve that is at fault. If as i suspect it is inside the bloiler casing, you will need a gas-safe boiler engineer to replace it.
There is a faint chance that the cylinder stat is at fault, but these go wrong approximately never, and 3-port valves go wrong rather more often.