Peter Scott wrote:

... tandem ...

This was the full range from small specialist shops to Halfords.
Do you know of an on-line supplier?

I'd try http://www.sjscycles.com/ - St John Street Cycles in Bridgwater,
who seem to have quite a thing for tandems, going by their ads in the
CTC (Cyclists' Touring Club) mag.

Also try the other suppliers listed on the Tandem Club's website -
http://www.tandem-club.org.uk/ - and their message boards.

HTH - Stefek