I've copied this from what I put up elsewhere in reply to a Senior not cocking, but the basic idea works with all the Webley over lever pistols.
"Ok. Here is what to do.
You have removed the mainspring? Put the guide back in and leave the barrel and linkage off the gun. Slide the piston back with a screwdriver or similar. Hopeully the piston should engage.
If it doesnt, then remove the guide and slide the piston a bit further back and see if it engages. If it now engages, check to see if you have the right piston in it. A Senior one should have a bronze ring as a seal. Mk 1's have a leather seal and Premiers and later guns usually have a Ptfe seal. All the pistons will go in but they will not work.
IF a new sear has been fitted at some point they sometimes need bit of the holding face grinding away to allow the piston to engage.
Assuming that the piston engages while you have the guide in, then try to push it forward a bit against the sear and the sear should hold.
If the sear does not hold the piston then either your sear is worn or the holding ring face on the piston-- Or both.
If with the guide in the piston does engage then that would suggest worn linkages/ cocking shoe, or the fulcrum on the cylinder being worn.
Usually bent pins are not the cause of a gun not cocking, but rather a symptom of people using excessive force to try to get the gun to cock.
More often that not, the problems start with a worn cocking shoe."