Finding the bonus cache for Sullington is easy; just find one of the two caches on the circuit that contains the bonus coordinates.
The bonus cache is an ammo box. Inside there is a host of goodies. This is mainly aimed at the younger cacher, as a kind of 'reward' for doing the walk. It's pointless calling them swapsies these days, as I have witnessed first hand the kind of 'trading' that goes on! So, you are very welcome to to offer youngsters a 'prize', but please, only one item each. Of course if you do want to trade, that would be great!
Please be discrete and please cover it up again, so that it can't be seen. Thank you.