Somewhere recently I saw an ad for a method of providing natural light
to an otherwise dark internal room - the concept was a light collector
of some sort mounted on the roof, with a duct or pipe to convey light
down into the body of the house, to a light panel or something in the
ceiling.

Sounded quite a good idea; I wondered if (a) anybody could point me at
a suitable website/supplier and

http://www.solatube.co.uk/ is the supplier whose tubes I have seen
installed.

(b) whether anyone has seen one of
these and can reccomend it?

Yes. They provide adequate light for most things, and if mounted on the
south-facing part of the roof they are a lot more efficient. Even on a
cloudy day in February they gave enough light for my nearly-blind
grandfather to move around quite well.

Or is it a gimmick which will cause your
roof to leak, and stop working when the (inaccessible) light collector
gets grubby?!

As mentioned by others, the rain cleans them. Just be sure the installation
is water-tight.