Thanks for advice guys it's appreciated, sorry not to reply sooner. Awesome job there murraymint!
I've put up a vid here how bad does that look ??
http://www.thequietplace.co.uk/emit/golfmk3.movFound a local guy who seems to be saying the right stuff, is a VW enthusiast, is talking £100 to patch both sides. He mentioned "sill covers" if the rot was bad, I'm not sure about this as I would have thought that's just hiding stuff, but he's coming over to have a look and talk properly I'm not sure I understood him correctly.
This is a common issue with the mk3, invest in decent trolley jack as they are much safer and very easy to use and avoid jacking on any of the sills or underbody panels, best places to jack a mk3 are under rear subframe mounting points just behind and to the front of the rear wheels and under the front subframe or front engine mount behind the radiator.
If I uploaded a picture of the underside could you show me where??
If I'd known how cheap trolley jacks are I would have bought one years ago!