Thank for replying.
The modern PSU is one from Poland sold by the ebay seller electroware_pl.
After the 1200 sitting on my desk for more than 36 hours of not touching it, as I left it the other day. Nothing plugged in apart from the modern PSU, video, keyboard, gotek and the ACA1233n. I powered it up, and again nothing happened.
I did as you recommended, and changed to yet another PSU. (I had tried the PSU that came with the 1200 as well.) I dug out an old heavy 500 PSU. I powered up, and the 1200 started working. I didn't move the ACA1233n around or anything. I soft reset, and held the two mouse buttons down, and it sees the ACA1233n again.
So in the off chance I tried the modern PSU again, and it still worked.
I've had this before though where it'll work for a while, then stop, then work again. I guess I'll look into seeing if I can get someone to recap the 500 PSU.
I will do testing at the weekend.
Thank you.