I would value it at around the £300 figure, due to the repairs needed to the stock and breech block, which needs some skilled attention, plus the pitting to the barrel and the apparent lack of any original finish.
I'm looking for a .22 myself, but I wouldn't consider this one even if it did come up for sale.
It's quite a late model and looks like it's had a hard life.
The last digits of the serial number are sometimes stamped on the lower front of the breech block as well as the places already found.
I posted a survey on the MK.1 rifle early last year, and it ended up containing a lot of useful information about the rifle, but for some unknown reason, it has now been deleted from the BBS
There are some pictures in my sig. site.
This is an extract from an article by John Atkins following the survey I did last year:
Although JA previously thought there were more .22s than .177s, Nos. 1 and 2 Bore sighted barrels appear in equal numbers - at least in this selection of some 85 known surviving Mk. I a/rifles.