Thanks chaps.
The grinding happens with a slack chain as well as a tight one (I initially thought it was a tight chain causing the issue).
Chainset isn't tight without wheel and chain, neither is the sprocket/freewheel. And the chain was fine before I changed the chainset and sprocket.
I am waiting for a proper chain whip to be delivered (my chain + claw hammer usual bodge isn't working) and I will put a) the old 16tooth sprocket on and b) the old chainring on the chainset and see if the grinding disappears. Thanks for your thoughts though.