Re the flaps and as per my ATPL course: When side-slipping, if flaps are deployed then the mean lift point moves in board towards the root. This reduces the stabilsing leverage and makes the aircraft unstable. Propeller wash has a similar effect. Apparently, low-wing aircraft are also generally unstable in the side-slip. High wing and swept wing aircraft are the most stable. If you draw a picture it is instantly possible to see the AoA differences and imagine how the airflows overthe fuselage and onto the wings. Intersting stuff.
However, I don't have the courage to try side-slipping for real without being taught. Where can I go in the Surrey/Hampshire area to learn how to do these kind of things?