Regarding the MD-Electronics SR decoder, here is the instruction manual (it's available in the "Downloads" section of the site):
http://www.md-electronics.de/sitecake-content/MD mXion DRIVE-SR.pdf
My recent move to DCC has been with MD Electronics and I've been happy with the equipment, documentation and email support so far. The only thing I'd note is some of the English is not perfect but is understandable - I occasionally augment it with my (terrible) German.
I can't talk specifically for the Track Cleaning loco, but in my experience with their decoders:
1. Is it a complete replacement ie remove all the old lgb pcb and wire directly to decoder ?
There is a section in the above manual that relates to it which may make more sense to you. For the locos I've converted, I've generally removed connections to the PCB and linked directly to the second for my own simplicity.
2 How do you control the speed of the cleaning wheels using dcc as I cant find where how do though it says you can
It looks like CV 130-135 are related to the cleaning functions. If you're unsure, a quick email to the team and they'll happily clarify. I've found them to be A+ in terms of support.
3 Do you know of anyone who sells these in the uk
Unfortunately, not. I know these are made/produced specifically by MD