Hopefully not the new bike?
No definitely not. Turns out the end screw had worked loose so just needed tightening. Halfords just tried to rip me off by saying they would only replace the bottom bracket if they changed my cassette, chain and chain rings as well as it was company policy. BULLCRAP. There was absolutely nothing wrong with the cassette.
We've had better luck with Halfords so far, including spotting a repaired bike's headset was loose (not part of reapir work!) before Chris took it out for a ride!