I decided to 'cheat'.

I opened/synced new wallet, and generated a new deposit address. I copied this down somewhere.
I then renamed the appdata folder and synced the old wallet.
I went to the swap tool, and followed the instructions given.
However, when I went to the old wallet to send the old ERCs, I discovered a new problem.
The balance in my wallet is correctly stated on the Overview page.
At the Send Coins page the balance is stated as Zero!! I thought this might be a minor bug, so i tried sending my actual balance anyway.
It did not work, as I got the message "The amount exceeds your balance". This, despite me leaving off the last few ERCs to keep the sending amount
below my balance.
I would really appreciate some help here.