:-) heya mate

It went like this (the reason I ended up all-in)

I raised 400 preflop
He called (everyone else gone)
Flop 667
I did a continuation of 400 (leaving me with about 600 or 700 chips)
He Insta-All-inned - no hesitation at all. I'd watched him play (he was on my immediate left) and the way he played, well, this looked like a stone cold bluff, particularly with the instant reaction, so I called it.

I was wrong :-p

I assumed he called with another high speculative hand like AK, AQ, AJ. I never put him on a high pair as I expected a reraise from him preflop.

Can't totally explain it but I was 100% sure he didn't have a pair (I was right, unfortunately).

As I put him on high unpaired cards I felt safe calling.