Hi Old Bull,
What a cracker!, thanks for posting the pics
Has Ollie, and Torrens have already said, it is a Marck Gem, but it's difficult to tell which model it is without knowing the weight and length?, even then it may be slightly different from other similar models...but from the shape of the breechblock it looks like a standard model No1 or No2, these were available in 4.5, 5.5, 6.5 and even 8mm calibre!
Marck Gems have been seen with patent marks from 1888 (Hiller - page 132), yours looks like it has the "DSE 3.12.91" patent stamp on the T-bar?, so it'll be after that date...and the factory was still open into the 1930's so it is difficult to date it accurately. The number 24 indicates early but then these are nearly always seen with a low-ish numbers so I'm unsure if they are actually serial numbers (all parts will be stamped with this number) I have two No2 models that can be fairly accurately dated to the 1920's, they carry the numbers 45 and 47 in the usual places, but they also have the numbers 6450 and 1634 respectvely, stamped on one of the flats of the octagonal barrel, so the numbers are ambiguous...
For more information on these, including pics of other Marck Gems, old Marck catalogues, etc, have a look at the link in my sig.
Cheers,
Dave