Just had the first estate agent round. He reckons that a newly built house in that position, built to a high spec and good finish, should go for around £550 -£600K. He reckons you could be looking £200K+ to build it. He reckons I could put the site, with planning permission, up for sale for £250K and get somewhere near to that. That's looking to be the more attractive option to me at the moment. I suppose I could put it up and see what happens - don't have to sell if I don't want to, do I. Then again, I don't need the money and I could either carry on working at the site or rent it out - I have three years before the planning permission expires and I'm assuming I could either renew it or re-apply in the future - the local council definitely want to get rid of a commercial property in a residential area so I don't think it would be a problem getting permission again.
It'll cost about 500 quid to put a footing in, at which point the development is commenced - planning expires if development does not begin within 3 years
Looks like your estate agents figures back up the "rule of thirds" reasonably well, particularly when one factors in the optimism that an estate agent brings to any aspiring vendor..