Knock sensor on most vauxhall engines is at the back of it, its about the size of your thumb, and is bolted directly to the block.

The bloke in the garage has been smoking some REALLY good shit if he genuinely thinks its more likely to be the ECU!

VERY easy to change, just a single 13mm bolt in the middle of them (well on the 2.0 16v i assume the smaller motor is the same or very similar) and not expensive to buy.

DEFINATELY try the knock sensor first before buying a new fucking ecu!