Question for older British people...

I'm British myself, but only 30. I saw this very good film - not for the first time - tonight, but I realised I might be able to get a question answered about something that was bothering me here.

I was curious about the van in which Alfie had his X-ray test. It had "NHS Mass Radiography Unit", or something very like that, written on the side. I've never heard of any mass campaign for X-raying people in the street in Britain. Does anyone know what this was? Maybe it's just entirely fictional, but I'm guessing, knowing British films, there's some basis in fact.