Rolson 50879 300 mm Combination Square Set - £5.73 @ Amazon

Great price. Hope the quality is ok. Thanks. 

Bought one took the chance that the quality is good. Great price.

This is so frustrating, only yesterday I was trying to find the centre of a metal disc and was going through all sorts of faff to figure it out... now I see this I realise I have one in a drawer and the centre head would have made things so much easier!!

This a decently accurate tool, the stainless steel ruler usually ensures the right angle stays at 90 degrees. I'm not as convinced about the protractor section though, the two brass screws that are meant to tension the rotating section don't do a very good job. Good for the money though. There's often a Bahco version that pops up at a decent price, that would be my preference if it needed to be spot on.

£10.62 now.