Hi Mark
I have reprofiled a fair few wheels over the 20 years, I have had a couple of articles published in the S7 newletter, unfortunately I can't tell you which ones, probably about 10 years ago.
Slaters are a little more difficult to reprofile because they are a composite wheel and the nylon only has limited strength and temperature tolerance.
Ideally they want to be very well supported when the new flange is formed. What kind of lathe do you have, ideally you really want something Myford 7 size, but smaller lathes will work for smaller sized wheels.
This is a useful thread, there are quite a few good photos further down, showing how to hold the wheels.
Re-gauging Heljan locos to S7
I'm a big fan of soft jaws, these are jaws made of mild steel that you can machine on the chuck so guaranteeing concentricity. If you can get some I would, they are not particularly expensive and last a very long time.
Make sure you use the latest S7 tool, the HSS one in the photos in post 5, these are much easier to use and give much better the results.
My preference with slaters wheels is not to take any material off the front of the tyre. Take it all off the back. You will lose quite a bit of the flange doing this, so to create the new one will require going deeper into the tyre with the result being a slightly smaller wheel that you started with.
You will though avoid any risk of damaging the front face of the tyre which is pretty thin to start with and it is very easy to take too much material off and ruin a wheel.
Richard