John Stumbles wrote:
Are mains and low volatge halogen bulbs the same size? If one has a fitting
which takes a mains bulb (by a spring clip around the outside of the conical
section) would a LV one fit?

(I know the connectors are different. The reason I ask is a client wants a
particular rather nice looking light fitting which seems to be available
only in mains, but wants them for a bathroom, so I'm thinking of chucking
out the mains lamps and fitting LVs.)

Probably not a good idea.
The current for a LV fitting will be some 20 times that of a mains one,
and the heating for a given contact resistance 400 times.