Hi, as mentioned above the stock is off a Webley Hawk, value around £35+
the piston is likely to be out of a HW35 and if the seal is in good order should fetch you £15+
The sights are indeed HW ones and will fit most HW rifles, a good set with the elements should get you £30.
The small piston seal looks like a HW77 size, if it's 25mm it'll fit the early hw77 and 97, if it's 26mm it'll fit in the later 77, 97 and 95, 98, 85 i'm informed.
The larger seal is likely to be off a HW80 or modern HW35. These should be worth a couple of quid each i'd think.
The barrel and cocking link assembly could be off an early BSA mercury (2 piece linkage) if you look underneath it'll have a serial code something along the lines of GB567 etc, if it has let me know and I'll tell you exactly which model it's off. Pricewise £35 tp £40 with the sights, possibly more.
The final item is the cocking link and underlever off a BSA Airsporter, I'm not sure but these are different on some models but it looks like a later one, MK5,6 or7 perhaps, Gotta be worth £25 to replace or repair a broken gun.
Hope this helps.
PAUL