Hi Graham, welcome to the board.
As Matt pointed out, I have two of these and love 'em. I had been on the hunt for one for a couple of years and have had a lot of luck with mine, because they both came to me within a short period of time and for a great price.
In good condition and with the original hardcase (the brown one with green interior, rare as lefty and it only fits lefty Ibanez Musicians and Roadsters), I have seen lefty Musicians go for as much as 800 euros (around 675 Pounds) but sellers sometimes ask even more. Rumour has it only around 350 have been made. Righthanded models can be had for around 450-500 euros in perfect condition, but because of the rarity lefties are usually more expensive. I paid 400 euros each for mine, but I had a lot of luck with my '81 and my '83 was in very bad cosmetic condition.
If you don't mind shipping it internationally (which can be a pain in the ass with neck-through-body basses because of size restrictions with many shippers) I'd recommend advertising your Musician right here on this forum, on
www.leftybass.com (the website is German but is visited frequently by lefty bass players from all over the world. Just email a photograph and the text of the ad to arnibi at gmx.net) and maybe also in the lefty section of the UK board
Basschat.co.uk, but they have changed their policy over the last year and advertising is now no longer free of charge (7 Pounds for a one-time ad, 20 Pounds for a one-year subscription). I'm not really familiar with other British advertising websites, except maybe Gumtree.
I hope this helps
Good luck!