Sleeves, we had it done in 2004 (it also began in May). The cost for the shell was £58k but our quotes ranged from £45k to £69k as far as I remember.
Everything else was extra - kitchen, granite, tiles, bathrooms, replacement windows to match, patio etc.
We had about £2k worth of steel included in the price because building regs said we needed it.
They trashed our garden - our friends had a similar experience and were gutted about it afterwards - if you are bothered about your plants, try to relocate them - and factor in the cost of making the garden tidy again afterwards. It might not happen but we had to completely re-turf ours.
Also - hire a portaloo - five builders using your toilet is not funny.
And make sure you think of every detail to be included in the quote - for example, exactly which light fittings you want (otherwise your kitchen will probably have a tasteful striplight...) and light switches, skirting boards etc. There are so many little extras to account for and our builders didn't point them out beforehand.
One last point - sell any reclamation yourselves. Our builders demolished a 1930 shed and (unbeknown to me) sold the bricks and roof tiles to reclamation for a small fortune (in the region of £600) which they pocketed. At one point, I came home from work to find our dining room radiator in the back of their van - one of them had assumed it was for scrappage when in fact we had removed it to paint the wall - they had tossed it into the back of the van and damaged it because they were looking to weigh it in for scrap.
(My father in law used to deliver kitchens and remove the old kitchens - he always weighed the taps and radiators in for scrap and used the money as his holiday funds!).
My builders weren't cowboys - I was pleased with the finished extension - hut had no idea of how they operate.