Anyone do it or done it and can recommend a route for an MTB, don’t mind the towpath if it goes all the way.

You can ride most of the way along the tow path. There's a section at Windsor, near the castle which you cannot use, but just cut through Datchet and you can rejoin it at Old Windsor. Some of the Tow path through Staines is a footpath, but provided you go slowly it's ok.