Having driven back from Stonleigh in the pouring rain then the rust could have been recent. I did check the car over before I took it on track and I think I would have noticed it then. I'm thinking nasty pot hole somewhere got me, or maybe it happened on the track, who knows. I'm just thankful I found it in the garage not in the middle of a high speed corner with a lorry coming the other way.
On initial inspection it looks like the last 20mm of the tube is welded on and its the edge of that weld thats gone. Not had a close inspection yet though. Off to a mates tonight to see if he can weld it back up until I can find a replacement/new one.