We have an old Celtic FF boiler which is no longer heating water.

The heating system is fine but when you turn the hot water tap on the
boiler no longer switches on. When the heating is on and you turn on
the tap the boiler turns off.

Anyone have any ideas what the problem may be and if it is repairable.
We were hoping not to have to replace the boiler until we renovated the
kitchen.

Also if it is repairable would it be an expensive job if getting a
plumber in?

Any advice appreciated

Thanks

Moira