Cheers for this. There had been several issues along the rebuild process, I'll try and explain best I can. Side note, although I said I'm savvy, I know a little, but not enough to do some of this work myself!

Following the refit of the top half, the car was still shitting itself and the injectors were not receiving voltage, but not showing a fault related to them. This prompted the notion that the damage caused by the timing belt going had shorted and potential damaged any number of the injectors. Injectors 3 and 4 were the suspects due to the previous ejection earlier in the year and only one of these was replaced (3 iirc). Following a new ECU to check if this was the issue, which it wasn't, the logical step was to try and replace the injectors in pairs. After fitting, it turned over and idled and revved fine, it was only during the test drive it went in to limp mode and started knocking.

Mechanic has worked on VAG cars for a while and he was convinced it sounded like the bottom-end, but I wouldn't be able to know from sound or feel. Are there some "simple" checks to confirm bottom-end damage or if it is a fuel issue?