Hi Stuart, the amount of yellow can vary a lot, on male bees, and books often only have space to show the most common colour form.
Have you had a look at the colour variations in the interactive keys on the Natural History Museum website. http://www.nhm.ac.uk/nature-online/life/insects-spiders/bumblebee-id/flash-version/
or the photos on the ID help pages and Species gallery at bwars.com (bees, wasps and ants recording scheme)
It was the bright yellow flashes, at the sides, above the white tail that made me suggest vestalis.
The photo of Louise Hislop's (on left side second row from bottom ) on this page: http://www.bwars.com/index.php?q=bee/apidae/bombus-vestalis is very like your bee with some yellow banding at the top of the abdomen.
Wow, thanks for all of the info!
It looks like 'vestalis' and using the NHM interactive bumble bee guide 'terrestris' also came up, it was a large bee.
I'm on a steep learning curve with insects, as you say so many different colour forms for some species but hey, it makes it interesting!