Hi Matt,
Yes I can see how the 'total debits and credits' figure is confusing, but the figure of £3,600 is correct. It is really just the total of all transactions - for example, if you borrowed £10,000 from the bank and then repaid it, the total debits and credits would be £20,000.
By the way, you don't need to create a purchase invoice when recording the mileage expense. Instead, you can record a Money Paid Out transaction with the Type set to 'General Payment'. Here is an example:
http://www.solaraccounts.co.uk/help/how-to-record-mileage-allowance-payments.php
This page talks about mileage allowance payments to company employees, but the transaction would be similar in your case - just set the Paid From Account to 'Owners Drawings and Personal Use'.
Also, note that VAT does not apply to the full mileage payment amount in the case of company employees. I suspect this is the same in your case, but I'm not sure - I recommend you check this with HMRC and your accountant.
Regards,