Chainline noise - videos/photo

It's a no-brand, i tried using a screwdriver and hammer to get the lockring off earlier to try a different sprocket but no luck, I'll need to use the proper tool in the shop on the weekend :-p

you need a mahhhusivvve punch and a big hammer.

but I think the sprocket may not (as Pifko said) engage well with the chain. Sometimes low quality cogs have cheaply cut teeth which don't set well into the chain links and create a little noise as they hit the linkages.....
i presume you bought a cheap 20t from ebay? There are some on there at the moment for as low as a fiva which are of dreadful quality, the teeth are hardly even half deep as they should be.

Check the length of the teeth on the sprocket, if they don't set into the chain properly and hardly have any length to grip them it's worth investing in a proper sprocket.

maybe post some photos of the sprocket?