Ok mum's boiler is poorly.
There is a leak. I have narrowed the leak to a white piece of plastic.
Problem is:
the company we called are unable to provide with a spare. So I have to do it myself.
But I can't find the name of the damn thing. Boiler is a chafeteau et maury (yes I know it's french cut the damn sarcasms I am after all in France) and here are some pictures of the boiler.
Front: the faulty part is the white one with the blue knob on it
[img] 
[/img]
underneath
[img] 
[/img]
semi-zoom
[img] 
[/img]
that looks one cheap and nasty boiler!
think it is a divertor valve of some description, good luck tracking one down.
get the model no/name and start googling
at least with a manual you might get to know the name
gusamc, that's done I have two manual a uk one and a french one. However it seems I can't find this part on any of them :s