I didn't realise you could still access the original split poly tool, so thanks to you both for that. Got a couple of buttons for it on my custom shelf now and will try sticking to that and see if any issues pop up the same. I had the same issue with the interactive tool a few places in the model where I know there shouldn't be any issues so guessing the interactive tool is a little temperamental. I eventually got around it (before reading your replies so couldn't try suggestions) the long way of using the tool to build additional geometry around the area and merging vertices before re-positioning. A very long winded way I'd like to avoid, so hopefully won't have any issues with the new old split tool.