I seem to have got to the bottom of the issue, eventually, with my Malaysian bank.
Despite clear instructions to the contrary they have not been paying your charges every month (but have on some months strangely) (hence the underpayment) and in September their 'system was down' so my direct debit came out a few days late and therefore it appears to have been paid on:
2nd October
29th October
29th November
31st December
My statement shows no payments for November but a double payment for October - obviously the payment on 29th October was for the 1st November so I don't know why the statement does not reflect this?
I have clarified that we pay all charges so there should not be a shortfall again. I couple hundred Baht when I am over next if you want - the cost of another transfer would be 4 times the cost of paying the arrears - is this okay?