It really depends on the amount of shooting the gun has done and how old it is, but as you say it is in good condition.
Second hand ones seem to be going for about £500-550 on average, if you post some pictures I could be able to give you a slightly more accurate answer, but without seeing the gun it is difficult, Small bores are still fetching a premium at the moment so you may get a good price for it, Have a look on guntrader.co.uk, and in the back on the shooting magazines classified ads to see what others are asking/fetching.
Regards
ss06
there is a picture of the 20 bore sorry its