Tibet in December - join a group/suggested tour agencies?

Hi there,

I'm intending to travel to Tibet in December. I'm a solo traveller, so am looking to join a group also heading there. I've had a look at a few tour agency websites and none seem to offer trips in December - I know it's the off-season, but from reading these forums, it definitely seems people travel there as part of tour groups at that time. Has anyone got any recommendations for tour agencies to enquire with? Or does anyone have a trip planned that I could join in on?

It sounds like some travel agencies can get around the policy of everyone holding the same passport, but in case that remains an issue, I'm a dual New Zealand and British passport holder (with my Chinese visa on my British passport).

Thanks in advance,
James