OK, not very exciting, but while having a look for one of these in Homebase today, I then found this on the net. Homebase were charging £5.99 for 10M extansion, cable only, no extras.
This is valid until 15/5/07
Everything you need to provide an outlet for a second telephone extension
Plug into an existing socket
Pack of suitable cable clips provided
Kits are approved for private DIY installation and full instructions are supplied
A range of kits containing everything you need to provide an outlet for a second (or subsequent) telephone extension. The kits simply plug into an existing socket and provide a new double socket at that point for the existing telephone etc. to plug back into.
Not for use on ADSL lines, for that use:
A53BB A52BB are 10 and 5 metre ADSL cables respectively