People are quick to condemn
Halfords out of hand, which is grossly unfair and not helpful to a novice.
No no no, people were very SLOW to condemn Halfords. It used to be a half-decent high street motor spares and bike parts chain, so was given the benefit of the doubt by so many people for so long that it became a marketing snowball that still takes in unsuspecting new riders, sometimes for a long time before they realise what's happening and bikes don't have to be that way. Halfords can probably continue for some time by feeding off new riders.
But now so many people have experienced the current Halfords workmanship lottery and been left taking their bikes to other mechanics to put right what Halfords got wrong, or have helped friends left stranded after their Halfords-serviced bike broke down, that a lot are now quite happy to label the chain in general with "avoid - ride away as fast as you can".
Many other motor spares/bike parts chains seem to have gone the same way, probably as part of a race to the bottom with Halfords. The better ones have given up on bike mechanic work, realising they're neither willing to join that race or able to command the slightly higher prices of a Local Bike Shop.
I'm not criticising the forumites as such, I'm railing against the automatic assumption that Halfords bikes are crap, and that it's better if their mechanics don't build them.
That's something else: I don't like many Halfords bikes, but if I was limited to them by a run-by-non-cyclists cycle2work or similar, I'd let them build it... and then get someone more reliable to double-check it!
Unless there's a clear smoking gun on the rear mechanism, I don't think the Original Poster has a hope in hell of getting more than that goodwill offer.
