Well it seems Bourbon IS group 2, which means it only gets tidied up rather than hard pruned. Leave it till spring, then remove any obviously dead growth, checking for live buds as you go so you don't cut those off. You can prune it down to 6 inches if you want to, but it will flower much later.
As to why you can't cut right back now instead of midwinter (for Prune group 3), if you did it now, the plant will respond by producing lots of new shoots because it's not winter yet - and those new shoots won't be hardened off sufficiently when winter arrives and you'd be in danger of losing the plant. You can, though, prune group 3 ones any time from December through to mid February, while they're dormant.

Thanks for your help I always seem to struggle with clematis......