He he he. Got you there!
Replaced the jets. The old carburettor's jets and emulsion tube had bigger diameter bores than the ones originally fitted to the new carb. Wazzed them in and put the thing back on the car. Started it up and it certainly idles better than when the old jets were in it. I've got the sizes noted down so I can look at a handy PDF someone gabve me a while back and see what engine those jets were meant for.