I bought a couple of cheap tyres for the commuter – a Conti Grand Prix and a Sport Contact 2. Which one goes where?

which one’s got more puncture protection?

that one.

sport contact rear I would have thought.

Dont know on road bikes, but on mountain bikes the general philosophy its to put the slicker tyre on the rear and a slighly more tready/grippy one on front.

This makes the front less likely to wash out, rear wash out is more easily correctable, and the slicker, faster rolling rear has more road contact to allow more efficient power transfer from the driven wheel.

on tarmac, ‘tread’ doesn’t add grip.

They both go on the wheels.

Will it make any noticeable difference at that price level? Should I have invested in Black Chilli?

for your commuter you should invest in tyres with puncture protection.