The Slammer hasn't really had any proper mainline duties since SWT finished using them (apart from on the Lymington branch) a couple of years ago. The Bluebell Railway has purchased the unit and it is to be used (I believe) to advertise the extension that is currently taking place between Kingscote and East Grinstead. It'll therefore be kept at EG until the Bluebell finally reach the town. After the extension is complete, I'm not sure what will happen to the unit, whether it'll be sold on or kept. I don't think that it will have much use on the line as only air braked locos can work with it, something that the Bluebell doesn't have AFAIK.
DavyCrocket or someone else maybe be able to shed further information regarding 3417.