From the pictures I can not confirm the year but what you have there is a Mercury/Mariner (so just a 9.9 Merc with Mariner paint and decals).
(my opinion) - this was part of a model run that went from mid 1985 upto 2012 (with a slight design change in mid 1994) and was the best built 9.9 2 stroke on the planet.
These motors have the best outboard ignition ever made (Mercury Thunderbolt 4), a great carb (Walbro), and a split block "headless" powerhead built by Yamaha exclusively for Merc.
I own 3 motors from this family (2 9.9's and a 15 horse), the oldest is now 27 years old and with some minor maintenance it runs as well as it did when I bought it new in 1986.
Now, as to price. On my side of the big pond, that motor, if running, and having only have been run in fresh water would bring about 1000 bucks (Canadian or US dollars).
Looking at the corrosion on the lower unit it certainly looks like this has seen a fair bit of salt water and the owner was not particular about flushing it down after each use (which extends the life significantly on a salt-run motor).
That is not a "show stopper", my 15 horse was a salt run motor in worse cosmetic shape than this one, but just about every fastener was seized up on it which led to alot of tedious replacements to get the motor "back in shape".
So for those reasons alone (plus you don't know if there is corrosion "inside" the water channel within the block or if the water tube is rotted away by salt) you have to consider that you may have to spend a few bucks (or pounds) to get her in solid working order. Therefore, I would peg the value somewhere in the $500 range.
Now I understand there is a premium for outboards in the UK, so given that, plus exchange I would put the value somewhere in 300-400 pound range.
If you were so inclined here is a before and after of a similar motor (paid $450 for the motor and spent about $300 to restore - 1991 model, and yes, it still runs perfectly today hung on the back of my 14 footer) -
