Hi Sakerman
The very first gun in THIS PIC is an S I paid £130 for it around three years ago
I think I was a bit lucky as it had just been serviced and a new rearsight fitted, it came with all the paperwork
Your father in laws should be worth more than I paid and at a guess , if it's in excellent condition i'd say £170/£180..
If i'm wrong (and usually am!!) someone will say so
John