I found Precision on Google. Called them on Saturday morning and they arrived 2 hours later. The work was done quickly and efficiently. Turns out that it was more than a broken spring so cost a lot more than I expected, however it was a very old door and now works better (quieter) than before. May or may not be the cheapest, but you can't beat the service!
Broken Garage Door Spring Repair
Garage Door Repair Springs