I had to stop the insanity...
That's how i feel.
But I've had another realisation. It suddenly occurred to me that a similar thing happened a year ago approaching the clock change. DS seemed to move his time BEFORE the clocks changed as though he knew what was going to happen. But then at clock change instead of his 5am WU becoming 6am it stayed at 5am. Long and short is I suspect it is not routine making for the earlier WU but a train going by waking him - a train that isn't there during the winter months

Yesterday he was like a zombie and yawning before breakfast

Then he fell asleep at 1pm in the car on the way home. He hasn't fallen asleep in the car for ages so he must have been totally and utterly exhausted. Transfer to cot, crying OT WU just under an hour later but I resettled and total nap time, wait for it... 3hrs

At that point he woke up screaming blue murder which continued almost without pause for an hour.
I'm now in a position where I have no idea if the issues are nap-drop related or train time related (prob a combination just to make everything harder to work out). We tried to find out last year if there was a new train going by at 5am but the information office wouldn't tell us. The only way to find out is to get up earlier to look/listen out for one.
In an attempt to compensate for the early train (if there is one) I moved BT to 7pm, he took 45 mins to go to sleep but maybe if I stick with it he will get that extra hour sleep, 7pm - 5am.
I'm drained, his mood is awful almost all the time and i can see it is all through tiredness. It hasn't taken very long for me to change from 'things are ok here' to 'this has got to change'.