Problem fixed. It was the big end bearing on cylinder 3. Metal shavings everywhere - both from the bearing and crank.

There looked to be very little damage to the block or pistons but no point machining the crank, getting new bearings etc so I purchased a 2nd hand EJ20 block with all the internals... runs great now

Thanks for all your help guys. I'll leave the video up - it might help someone out in the future.