I agree with Gill, definitely between the wars fashion. The couple on the right do look black, and maybe the child in front of them too? They would be a small minority in the 30s, Found this extract on a site about immigration
http://www.rootschat.com/links/07kl/(scroll down to 'First World War and After' - not good - hope the family in your photo were treated better)
Eastwood, try emailing your photo to some of the local churches to see if they have old photos/parish magazines etc still in the building - to compare mugshots of previous vicars. I contacted Rev. Ireland at St Peters a while ago with a marriage query and he was very helpful. http://www.levenshulmechurches.com/
They have shifted the actual registers to Manchester Library now, but they might still have bits & pieces in the church.
Good link to the Crockford's site Ken, never knew that was around, thanks!

Barbara