Just an update - new pipe arrived, cut to the required length, went in like a dream, still leaking. Before we (got my dad on the case as well) went to the length of taking the plasterboard off, we managed to get some paper towel wrapped around the flush pipe and flushed toilet. Paper totally dry, bucket of water down loo and still dry but leaking when flushed.
Took toilet back off wall (again) and had a really good look but couldn't see anything untoward. Stood toilet upside down and filled the manufacturing holes with water and water starts running out of the rim. Soooo, seems like there's some kind of connection/crack between the flush system and the body of the toilet. This was mentioned in one of the first replys but never thought about it being an internal fault. I assumed it would be visible. Off to send an email to the toilet supplier but just hope they aren't too funny as we've had the toilet for a few months until we got around to that bit....
