Just before O levels? Rubbish - it was just before playtime! And just notice who was the one not listening when 'Miss' said "Let's have a picture of you all looking at the castle." Yes, the one on the far left with baggy shorts and half-mast sock!
Didn't know they had 'O' levels for 7 year olds Martin. Do enlighten us. Which brings up the subject of why you even bothered with TH if you already had 'O'levels" in your pocket. Still, very easy to recognize you Martin!
Well found prefect, the 100Hz is down to the rotating speed of the prime mover and the number of poles in the alternator, must look further into why they used 100, and where else.