Thanks for the quick response HandsOnPaws, like I said before this is unusual behaviour for Tifa. It is only in the last month or so that she has occasionally started acting like this (maybe 4 or 5 times). I was told a quick tap on the nose was a good way to let her know that what she has just done was wrong but we will now stop doing this and just ignore the bad behaviour when it happens and use cheese (something we use a lot anyway to reward her for sitting, staying and fetching etc) to try and get what ever she has back off her.

I have read that she may possibly be going through a teenage rebellion phase?? could this be a reason for her behaviour?