I'd expect that a faulty flow switch would simply cause it to run
cold. I don't get anywhere near 5 litres out of it before it cuts
off.

Even better. I would expect the combi internally to be like a tiny heat
bank. I suspect that this keeps the heat exchanger to mains hot. However, I
would expect the heat bank only to be circulated when the flow switch
activates. This means that you will only get a short burst of hot water, as
the heat exchanger won't be replenished even from the 5L store.

Christian.