Hi Amanda
Unfortunately I am not in an ethnically mixed area, however I have worked with ethnic communities in London for many years in a former phase of life and I would consider:
Zenith Rahman at the amazing Bromley-by-Bow Healthy living centre in East London
Taste of a Better Future - Women's Environmental Network - involving women from diverse communities in growing organic food - tapping into wealth of experience from life before UK - see especially The Coriander club - social entreprise run by bangladeshi women (also London)
the amazing untapped skills of people used to remaking and mending - I once tried to find a job in a car body repair shop for a young afghani who could beat out a crumpled panel to perfection with just a hammer - skills not needed here - we just throw away a bent panel and put a new one in...
Refugee Council and Refugee Action - could put you in touch with key members of communities with experience of community action and global issues
Local college ESOL (English Language) teachers - know people from all communities who need to develop skills for employment in UK context (very often ESOL teacher is only friendly non-judgemental interaction people have with English people - what a resource Transition volunteering could be for building skills, trust + community cohesion!)- also try 'Into Work' training organisations - jobcentre could help?
Handsworth Uplands allotments - very diverse
West Mids Racial Equality - support any organisation wanting to address racial equality issues - the CRE in Stoke has a community Development worker - don't know about Birmingham... www.rewm.org
Good luck - a profoundly important issue both in itself and for the success of TTs in embedding deep social change :)