In message , Ben
Blaukopf writes
The boiler is a Glowworm 30ci plus, which has a 5 litre store.

If I turn on the shower or taps at a low flow rate (hot), then I get
water for a bit, and then the flow cuts off. There's no problem
with higher flow rates. Curiously the monobloc mixer I just
installed in the bathroom doesn't appear to have this problem.

Any ideas? Is it a case of 'they all do that sir'?

They do.
But it's not the boiler. It's the tap (some seem to be worse than
others).
When you switch on and adjust the flow down the tap washer is only a
very short distance off the seat. As the boiler heats up the water the
hot water expands the spindle in the tap. This causes the washer to move
closer to the seat decreasing the flow; at a certain point the boiler
flow switch turns the boiler off.

Having said that; I'm not sure about the shower. I didn't think their
internals were made that way.......

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