OP here. I attended a Nikon roadshow this morning and compared my 24-120mm f/4 with one that they had on show. The difference was striking. I have put up with a cr*p copy of a lens and have sent it to Nikon for "adjustment". As it is out of warranty, I suspect it will be between £70 and £120 as that is what I have had to pay before when a lens needed adjustment after the one-year warranty that we enjoy in the UK.