Hello Steve,
As you are probably aware, you will first need to obtain a Retention certificate for the number and this costs around £110 (or did) and will entitle you or someone who buys it from you to transfer it onto another car without cost, but the cost will be down to you to start. You will of course then need to buy another set of number plates which will return your current car back to what it was originally, but for £110 plus the cost of the replacement plates, you will have a number to sell.
Registration Transfers (and there are far more) will value the number for you and my experience is that that valuation will be "encouraging" and they will advertise it for you without charge and offer you the agreed 'price back to you' price. Fast forward some time (and in my case it was almost 10 years before I gave up!) and it is possible you will be asked whether you want to bump it up and pay an advertising cost or perhaps reduce your 'back to you price'. You may get - as I did- a discussion about a rock bottom offer from which they deduct the cost of the retention cert ( for which I held and had paid for!) - experienced sales person turned out to be inexperienced and that info.was wrong in my case, and at which point the interested purchaser disappeared! Now it's possible to have them advertise the number while it remains on your car and a prospective buyer then pays for the transfer. Sounds fine, but I think you will, find there would be very little left in the pot for you when all is said and done - to a not worth the bother level.
If it were mine, I would be looking on the DVLA website for similar format/layout numbers which can be bought straight from them at a price which includes the transfer fee and it would not surprise me if around £250 might be the buying price. So if it goes to Reg. Transfers, taking the £110 transfer fee, plus repalcement plate costs for your car plus Reg.mTransfers commission it could leave you with enough to buy a packet of !Removed!! I really would look at what DVLA are selling similar ones (including transfer costs ) for and go from there.
I appreciate this is all very pessimistic and Reg. Transfers valuation may appear to make it seem attractive to follow up, but.... Possibly worth thinking about why the previous owner didn't sell it. I guess you didn't pay a premium for the car because of the number (hope not) so I would take it as valueless (in real terms) and move on buying your replacemnt number. Having said all of that, it is just possible at someone may fancy it and be prepared to cover the cost of transfer plus some added funds. You could be lucky Steve.
Kind regards,
Gareth.