.... well I would look for s/h on ebay but not found anything as yet or else it goes for too much IMO (ideally I want something a bit battered to make it less attractive to theft as it is going to be my commute bike).
For example today - used Felt bike sold £ 345, factor in £ 25 petrol to collect then change from 8 to 9 speed - shifters and cassette, now compare to the new felt I listed that is new, delivered free and with guarantee for £ 370 so the new one works out the better deal.
I'm not quite with you...
You want a battered commuter, so why consider new?
OK, is the Felt in your example the same model? If so, it's a poor deal.
Why do you need to change from 8 to 9 speed?
£400 will get you a very nice lightweight
older bike with decent components that will outperform any overweight £400 modern junk. You don't even need to spend that much. Last year, I bought my daughter a Kona Kula for £170, virtually unused. New, it was over £1000, 10 years ago. It's a much better bike than anything you'll find for £400 today.
Give a man a gun, and he can rob a bank.
Give a man a bank, and he can rob everybody.