In that case I think the contra account is the best way to handle it.
So where you would normally invoice each other, you would pay it from a new bank account on QuickFile created for contra-invoicing.
For example, if you invoice them for £100, and they invoice you for £100, mark both the purchase and the sales invoice as paid from the contra-account and they cancel each other out. Where there’s a difference, the bank balance would reflect that.
The following posts may help: