Okay, Ive got some more details about my daughters boiler. Its a WB 28i
Junior That according to WB is 15 years old.

Currently itll only make HW when the thermostat is calling for CH and
often not even then. Frequently of start-up you can hear the boiler kettle
get and the locking out (almost certainly due to over-heat). I witnessed it
do this and couldnt detect any vibration from the pump whist this was
happening so I suspect that sometimes the pump isnt starting.

Also the HW temperature is hard to control swinging between too hot and too
cold.

To me it sounds like a dodgy pump and maybe a dodgy flow switch too but I
dont really have any first hand experience of combis. Given its age, is
it worth a piece by price repair or should we cut our losses and just go
for a new boiler? (Not a Baxi).

There *does* seem to be some sort of magnetic filter in the system so
whilst slugging might be a factor, Im not sure its a major factor.

Tim