I have an ex council semi. In the roof there is just one run of purlins about half way up the roof. Is this normal for a 1950's semi?

IF ITS A HIP ROOF ITS QUIET NORMAL TO HAVE 1 PURLON AROUND THE ROOF JOINING AT CORNERS OF HIP POLE THEN THERE ARE KING PINS UNDERNEATH PURLINS RESTING ON LOAD BEARING WALLS,HOPE THIS HELPS TO PUT YOUR MIND AT REST, TONY

Yes this is normal in a 50s semi.

Yes...Most houses usually only have one purlin, unless the rafters are extremely long (longer than timber lengths)/or are joined/overlapped and need supporting where that happens there wouldn't be any need for another. If the property has had a lot heavier roof covering put on from the original, sometimes extra purlins are fitted with additional rafters to take the weight of the heavier tiles etc. But usually there is only one...