In reply to Tobes:
These days changing a battery comes with a bit of faff.
Radio, resetting the windows (this took me half an hour the other day as one window just refused to reset and you can't use them until you do).
Some cars are also picky about how long it's been since the ignition was switched off to avoid triggering the alarm and immobiliser etc.
So they probably want 5 minutes to check the vehicle type for any unusual procedures, 5-10 minutes to change the battery, 20 minutes to sort out any faff that crops if after. If nothing, but of extra purohit there. So that's £40 in labour.
Or, just learn to do it yourself. It's easy, follow advice above about never working on the positive when the earth is connected. Save a bit of money, and then you can do it again for any future problems you may have.