I doubt that it's worth very much at all. (My own inclination would be to give it to Oxfam, as they have a specialist department selling collectable books, and they might get a few quid for charity from your donation).
I've done a bit of buying and selling of books (especially first editions) over the years and I'll offer you a notional guide to prices:
A first edition of a very collectable book, in good condition, complete with a dust wrapper in good condition might fetch, say, £100.
A similar book but with a slightly tatty dust wrapper might be worth £50.
A similar book without a dust wrapper (they're VERY important to collectors!) might be worth £20.
However if the book is obviously 'used', the value might fall to £10, whereas a 'reading copy' (which is very worn) might only fetch a fiver.
All of the above assume that it's a first edition. Otherwise its value will probably be somewhere between £2 and 10p.
Even so, the fact that the Abebooks website (which is now part of Amazon anyway) can only find one copy of your book suggests that it does have a certain rarity value, so perhaps the William Heath Robinson Trust might be interested in purchasing it?
http://heathrobinson.org/index.htm