OK, so I had a chance to investigate this further, here are the results:
Essentially, it's a vent hose routing problem. Either the were not routed properly from the factory, or when the dealer fitted my Termi high pipes, they may have removed the tank for access and screwed up the hose routing putting it back on.
1. So with the tank off, you can see the kinked hoses. They have be routed aft of the airbox rubber elbow, when they should be forward.
2. Here they are routed correctly in front of the elbow. The problem is you have to put them on the tank first, and as you put the tank into position, you slide the hose past the elbow. I use a bit of WD40 to help them slide past.
So I removed the tank fully, emptied it out of the old fuel, and put it back per the above. Tested the vent tubes with the cap assembly off and it's fine, put the cap assembly on, refueled it, and spilled some fuel on purpose on the drain/vents, it drains perfectly.
It's just something that will affect anyone who removes and refits the tank as the easiest option is to let the hoses stay routed aft, but then they bend on themselves and block.
By the way, I found the need for a special tool as per
this thread to be BS. I followed the workshop manual and pushed the connector in to release the hose fitting, and it came out fine. I made sure the tank was fairly empty first but it seems the fittings seal off when the tank is off anyway.
Here is the photo from the manual: