theradiotubby:
On or around the transfer date arranged by Eon, Eon will either take meter readings or more likely ask you to take and submit (to Eon) customer readings. Eon will transfer the readings to British Gas who will issue you with a final account.
The surplus will be returned to you by British Gas, possibly to the DD account if you leave the DD open, alternatively by cheque. Given that there is currently postal disruption it would be worth confirming what refund method British Gas employs.
When you do that they may wise up to what you are doing and offer you a competitive tariff and/or a partial rebate of teh £300. Up to you.