Thank you very much for your comment, Gareth. I've never had any experience with private plates before so your insight is very much appreciated. I did request a valuation from a couple of specialist services and both valued it at £700-800 (what I would get in case of sale after their commission is taken). The car has been in constant use (judging by its MOT history) so had been continuously taxed I believe and has a current MOT. As to why the previous owner didn't retain the plate for themselves - it's anyone's guess I suppose. Also, thank you for sharing your experience that it may take a long time to sell - this is something I obviously need to take into account.
Kind regards,
Rafael