This is the No.7 MkI "knife" bayonet (oddly falling into the "socket" category) originally adopted for use on the Sten MkV machine carbine but later (1947) on the No.4 infantry rifle as well; this therefore making the No.7 the true successor to the No.4 "Spike" Bayonet.
These were in service from about 1945 into the 1970s.
This bayonet - unlike the latter No.9 MkI - has a proper handle but could also have been used with the "broom-stick" attachments issued as with the No.4 "Spike" Bayonet.
They are very basic knife/bayonet, but the attaching method is somewhat bizzare. This bayonet has a swivel-pommel that turned 180 degrees and locked in place by means of a latch on the upper tang. The muzzle ring does not actually slip over the muzzle when attached to the No.4 infantry rifle; the socket actually then slips over the rifle muzzle and twists into place as with the No.4 and No. 9 bayonets; I did not have a Sten Gun in stock to check attachment method in that configuration!
These were not produced in large quantities and are scarce.

The scabbards are sheet-rolled-steel and fit virtually all of these later British "Bowie" bladed bayonets.