I don't know what the problem was but it looked like the bad old days of the terror alert some six months ago!
I do, I passed through security last week having removed my shoes, jacket, overcoat and still beeped. The very nice security lady told me that they had new scanners installed that were now doing the 1 in 4 selecting. All well and good but on that day the scanners were set to 3 in 4 and on Friday they were set to every other person. That is why the queues have gone mad again.
T3 gate management is done by BA BTW.