Another update.
New rain sensor was fitted on Thursday. The wipers kept going off as the man was refitting the trim round the rear view mirror, we found that as you moved the mirror, it set the wipers off. So, any vibration or bumps would be setting it off. I have since noticed that having music on at any considerable volume causes the mirror to vibrate.
I am now having a new screen fitted as the bracket for the rain sensor on the cheapo standard screen they fitted was not strong enough.
I have requested that they fit either a windscreen from the dealer, or a Saint Gobain screen (which is the glass VW fit in the factory). They have agreed to this, and are bringing me a Saint Gobain screen to fit on Thursday morning.
I've really got to hand it to Auto Windscreens so far. Whilst it's been a pain in the backside, they have done all they can to remedy the situation and they're really 'on the ball' with ringing me back. They have said that the manufacturer of the dodgy windscreen will be blacklisted on their systems to ensure this doesn't happen again. So I've got a new rain sensor straight from the VW dealer, and hopefully an OEM screen!
I hope this can help someone in the future with a similar issue.