If it's scanning and not asking for a code then it's probable that it's just that the ariel cable may have been disturbed.
Any half decent mechanic would have used a code-saver when disconnecting the battery so it shouldn't need recoding and it doesn't sound as if it does.
I doubt that Halfords would have a clue what to look for, if it was working before the alternator was changed and now it's not then take it back to whoever swapped the alternator over if you can't see whats amiss yourself.