Definitely post-war. Can't quite make out if it has a "Birmingham 4" stamping, which would make it pre-1958. Equally, not sure if the trigger is blued (early post-war) or just dirty/corroded but originally shiny (later post-war).

The "Birmingham 4" stamp is desirable, but only to the extent of a 10-15% premium at most, and some buyers don't give a monkey's either way.

I've never detected that the differences between the late 40s and later pre-58 models add any real added value, except maybe to a real anorak trying to build a comprehensive collection - and that guy won't be very interested in a pistol in that condition.

I paid something like £90 three or four years ago for an early post-war one in much better condition in a private sale, which I thought at the time was an OK price, though not a bargain. I doubt that any Mk1 in working order with no parts (eg grips) breakages is worth less than £50 (and, sadly, one could probably get more than that in aggregate if broken and sold as spares), unless very badly rusted.

So, my advice, assuming the thing works OK, is that a sensible asking price would be £50-60. Half an hour with 0000 grade steel wool and 3-in-1 and someone has a nice plinker that won't gain much value, but won't lose it either.

By the way, Nige seems to have more luck than I in finding items that range from good value to bargains. A "superb straight grip" for £100 is not easy to come by these days. And I disagree with him in a friendly way: the Premier and Senior are nicer than the Mk1.