No hot water in bath when heating on?

[purpleme12]

I moved into here in the summer
Hot water always really good
A couple of weeks ago it started getting cooler in the bath I could tell. Still warm but would be too cold to stay in the bath after a while.
I think I've figured out now it's not hot when I've got the heating on!
I don't some experimenting tonight and when I turn it off again the water seems to go hot again
Can anyone shed some light on why this is happening?
I'm worried

We had a similar problem and it turned out to be a partial airlock (if you can have a partial one) in the hot water pipe.
I think they connected a hose between the hot bathroom tap and the kitchen tap and turn both on and the mains pressure from the kitchen tap pushed the airlock out.
I don't know if one should really do this but nothing exploded!

Find an engineer online who specialises in your boiler brand - eg baxi, vaillant and they will fix it quickly and reasonably hopefully