Radiator heat, but hot water disappears after about 1 minute

I have a Boilermate II installed upstairs and after some loud banging noises as it & some investigation I found that the top-up tank was empty and refilled.

British Gas visited and
a) diagnosed a pump failure (which they replaced),
b) turned up the pressure in the system from 2 to c. 3 I think and
c) unblocked the filler-pipe from the top-up tank into main unit - coat hanger I think.
Main tank filled up and full hot water for a few days.

However, I am now in a situation where the central heating works fine, there is water of some warmth in the top-up tank, but that on turning on the hot tap you get a burst of hot water for c. 1min and then it goes lukewarm for a long time if left running. You might be able to wait an hour and then repeat.
You can see that the downstairs boiler is kicking in after the tap has been running for a few minutes.

So similar symptoms to another post in this forum, except I do have central heating rather than no central heating.

I'm at a bit of a loss. One could speculate that the main tank was quite short of water reducing the amount of immersion that the hot water coil has thus reducing the time available to heat water, but if that was the case I would expect the expansion banging noises to return as in the initial problem at the end of summer

BM II
No hot water.

It will be
Mixer or blending valve
Or coil scaled up inside

Mike
MGC/H2O
Norfolk

i.e. I have had hot water for nearly a day.

KitKilgour wrote:i.e. I have had hot water for nearly a day.

Just wondering, was the CH running when you were having the hot water problems? It is better when the CH is not running?

SID - I did check - 6am bath still ran cold, even before the CH was on. It does seem to be fixed now (fingers crossed)

SidDIYer wrote:

KitKilgour wrote:i.e. I have had hot water for nearly a day.

Just wondering, was the CH running when you were having the hot water problems?  It is better when the CH is not running?