I currently own an 11 plate Grand C-Max but lookingbatvS-Max for next car. Seen a 17 plate for 10995 with 118k on the clock but hasn't been used for 8 months and the thermostat is being changed before it gets a new MOT (which ran out in August). 

I've only been driving 5 years and not sure how good a deal it is? If anyone could help it would be greatly appreciated 

It's your money, but at that mileage, no way

A quick glance at AutoTrader shows a wide range of asking prices for 2017 S-Maxes from under £10k up to nearly £20k depending on mileage, spec, etc. 

it might be helpful to mention whether you are looking for petrol or diesel, what power output, manual or auto, high or basic spec, etc.

Why hasn't it been used in 8 months?  More stuff broke on my car in the first few months it was parked than the 3 years I was driving it.

Used car prices are coming down now, after a few years of being excessively high.  There's no way we'd have paid 5 figures for a 7 year old car with 120k miles pre-covid.  But, it is what it is now...if you need a car now and can't find anything better for the price, then it would seem like good value.

Might even be the same car, but I spotted one in Kent at £10,995, 100k miles, 2.0TDCI 150 manual, so that seems to be about the going rate.

https://www.autotrader.co.uk/car-details/202307089432551?sort=price-asc&advertising-location=at_cars&make=Ford&model=S-Max&page=2&postcode=YO26 5QD&year-from=2017&year-to=2017&fromsra

(That's not my real postcode in there, btw!😀)

MOT history is weird on that one.  Hasn't been driven for a couple of years.  Failed MOT on airbag light back in May '22 and wasn't retested for 3 months, where it then failed on headlight alignment.  Was then passed in Aug '22 but MOT has lapsed again since.  Suspect something wrong, potentially an intermittent fault.

Cambelt is also due on mileage if not already done.