Help and Support ...or lack of...

I migrated away from PlusNet on the 12th September to Sky. No problems getting my MAC code etc... all went ok...

but..

My direct debit for the month 10/9 to 10/10 went out and so I was charged

The account gets downgraded to a free PAYG dialup when you migrate out. This means you will still be able to collect mail.

If you asked for your MAC on 4th September then you will have started your 30 days notice then. This ends on 3rd October and you are liable for the connections costs until then, whether you can use them or not (the date of the migration isn't set by Plusnet but by you and/or the new ISP so it could of been set for 3rd October to avoid paying for any unused service). You are due a refund for the 4th October to 10th October period and this should be processed over the next few weeks if you have requested it (which it sounds like you have).
You may get more than just the 6 days after the 30 days notice but you are not due it under the T&C's governing your account, so it would be a 'goodwill gesture' from Plusnet if they paid more than that.