Thank you for responding - apologies for the delay in getting back to you as I've been away (hence the question...)
On the auto response form, I did set the start date as well as the end date. Indeed, when I went into the page again, those two dates were correctly set as I had entered them. However, as I was 'being prepared', and setting a start date in the future before I went away, I realised that the auto response actually started from the moment I pressed the save button.
Solution in future would be to only sent an auto response at the point of need, but it would appear that something is not quite right when setting a start date in the future.
