Bodywork is EVERYTHING on a mk1. If it's all good with no signs of rust (wings, sills*, base of 'A' pillar, top of windscreen are all prime candidates for rust) then it could be worth up to £1500. If it's even a bit dodgy in any of those areas then you might struggle to get £500 for it.
*pop the skirts off and you may be surprised by just how grotty it is behind there.
/EDIT - Just looked at your listing and you've barely mentioned body condition. If it's all good then make a big deal of it, it'll have a huge effect on what you get for it.