Your air-rifle in question is an Air-Arms Hi-Power.
Air-Arms did take over Sussex Armoury, however it was NSP engineering in between then the name Air-Arms came from them.
You state that your stock is shorter than Sussex Armoury Stocks. They are however the same size except for the Sussex Armoury Parabellum. Also the internal part of the stocks are different, so you can't put a Sussex stock into an Air-Arms Action & visa versa.
Here's a >LINK<
to my collection so you can the simularities.
When yours was manufactured could be between early/late 80's. I did contact Air-Arms before for manufacture dates of mine but I never got a reply.
You can try Chambers gun spares. As they do a parts diagram however the diagram is of the woodstock version, the action is the same.
For tips of servicing/polishing I'm not up to that level of experience.
Hope That Helps