Doctor Who 70s book

It is London, 1978, and tensions are high. Seema and her family are struggling, but she has learnt to keep her head down, not create trouble. That is until she and her two friends, Terrence and Inderjit, decide to join an anti-National Front protest in the East End. And when trouble does inevitably find them, the friends are saved by the appearance of a myserious, seemingly broken-down bus. But inside this bus it is like nothing any of them have ever seen. It is a journey through the most wonderful landscapes, where visions of hopes and dreams enevlope the lost group. Who - or what - is this strange place? The tall, grumpy man with white hair might know the answers, but then he also seems just as scared as they are . . .