I have two for sale. One stubben and one Bates GP. Stubben is a fab jumping saddle and in great condition. PM me if you are interested. Both are 18". The stubben is a medium/wide (31) the Bates in on e-bay at the mo though.
Otherwise best to find a good saddler who stocks second hand saddles, although I've never had a problem buying on e-bay, just tricky getting the right sized saddle.
Im not to sure where you are based but there are two places i would recommend but you would need to go to them as they are not web based, i notice you say east anglia one is in the colchester area and one in hertfordshire both carry a good range of secondhand saddles at good prices, not sure if im alowed to post the names of the places so if they are removed then please pm me.
Sandon Saddlery Sandon Herts, I can get directions for you and the other is Priory Saddlery Boxted Colchester, although some saddlers will bring out secondhand saddles for fitting.
Do you have a price range?