Did you resolve this, trt?
I know we've had something similar a few times and, as in claryS's case there turned out to be a simple explanation, although we did have one rogue transaction in the Far East some time after a holiday in Hong Kong.
It may be one of these recurring (annual) continuous payments that you read about where you thought it was only a one-off purchase a year ago. Or it could be an old such payment from many years ago has been reactivated in error- there was a similar incident on Money Box (radio 4) a few weeks ago