Not a lot I am sorry to say.
If it really is in "exceptional" condition (no distinguishable marks) then it may fetch from £35 to £50. Other wise if it has (in ebay speak) "normal wear commensurate with age" ie; looks like it has been kicked about a few times in the back of a van then it is probably worth £10 to £20.
This is an example of what I would describe as in the middle as just "good" condition, and possibly worth about £20 to £30.
http://www.kbmuseum.org.uk/kb_images/KB_FR10.jpg
Depending on the month, weather, number of bidders, oh and of course if postage is offered.
Mike