I have had a few of these and they are relatively easy to service, however, if you have never done one before, they could seem a daunting job to undertake. they can easily be screwed up by the novice and never seal up again.
Take a look at the diagram available here >
LINK
It shows the positions of all seals, problem seals are often the Gauge seal, the o ring on the cylinder fitting ( OR144 on the diagram) and the fill valve seal (OR068) and the bonded seal (BS118)
you dont need to fully dismantle all the rifle to get at those which is good as you dont need to mess with any anti-tamper devices which may be fitted.
Certainly worth trying those before anything else.
Paul
EDIT: I meant to add, take no notice of the diagram having the wrong stock shown, thats the Webley Spectre offering which is the same metal parts. Getting the stock off the scimitar is a pig a times and you have to remove the cylinder off the gun, then remove the rear stock part, then remove the grip ( careful, the top of the grip will crack if you dont remove the screw that goes through the pellet probe) Then and only then can you remove the bottom stock section. Sounds worse than it is, honest gov...
If you get stuck, PM me and I'll see if we can talk through it?