That reminds me: Anti-inflammatories are wonderful things. As of this moment, nearly full range of movement back, been steadily improving since Thursday evening. Done some tentative stretching, couldn't quite manage the bike yet but I reckon I might be able to go for a run tomorrow. So just a muscle spasm. Well I say just but it's easily the most pain I've ever had in my back. Thanks everyone for your support and sympathy, it was most helpful on Thursday when I was feeling pretty low.