If it is a recoilling springer, then it is probably a Model 5. Otherwise it is a model 6 or a model 10, these are more complex and have recoilless mechanisms.
Have a dig around on the Chambers website, look at the parts diagrams and see what you think it might be.
The Model 5 has a very, very long production history and is a sturdy, accurate design. They are still making them now, and I think the early types date from the 1930s. Worth fixing up and using as an informal target shooter or plinker.
Like this one is it?
http://www.gunspares.co.uk/shopdispl...id=24392&cat=5
or this later one ..
http://www.gunspares.co.uk/shopdispl...d=24397&cat=5G