Intermittent Hot Water

Hi, I hope that someone can point me in the right direction.

We have experienced this fault years ago before but it seems to have come back. The plumber at the time replaced a sensor but I do not recall which one and cannot contact old plumber. New plumber seems to struggle to fault find.

When I cycle through the temperatures I get 77,41,59 which seems about right.

When I turn a hot tap on I get 77,25,30 which is logical as the water temp coming out is not hot.

If I turn the tap off then the third temp rises back to 60. If I turn the tap on as a trickle then I get hot water at this temperature but it is a trickle.

All the time the tap runs whether at full pelt or slowly there is no call for the boiler to heat any water.

So my naive thing is that I get water heated during the times that my boiler fires for the heating. I then am able to access this water but only at a trickle because the system is not calling for heat.

If T1 does not fluctuate then does the fault lie here and is it a temp sensor or a pump problem.

I can normally get a half decent shower in the morning on low flow but it is pretty much it after that.

Sorry if my explanation is not clear but I would appreciate any assistance.

Thanks

David