Tom: Yes, you've probably hit the nail on the head - if you read the early part of the thread http://forum.vodafone.co.uk/t5/Samsung-Galaxy-S2/SGS2-GT-I9100-GPS-issues-taking-ages-to-get-a-lock/td-p/925317 you'll see a few other people asking that, and those who have rooted their phones say that appears to be a solution.
So you're right - but some of us are uncomfortable rooting things, haven't got time, or both, and we're waiting for an official solution. Of course, if there isn't soon an official solution, that may have to change.......
I didn't want to root my phone because I thought it would prevent future support from Vodafone, but having seen what their support is like, I'm starting to think I'd be better off going it alone anyway.