Excuse the abbv language, want to avoid TLDR

Had wii, played Caleba's Dangerous hunts fine
Upgraded to Wii U and didn't play it for a while
When I try now the cursor is fine in menu (even the game menu) but when I start playing or calibrating it is extremely sensitive and flies to the corners with little movement.

What's changed since the I played on the wii?
In the same TV cabinet and on the same multiplug...
- Different HTPC with onboard BT & wifi module
- Fibre

What have I tried (all with the same result)
- Using multiple different remotes (all motion plus)
- Multiple sensor bars
- Turning off room the lights (which apparently I could easily use instead of the sensor bar)
- Testing on a wii again
- Unplugged TV, AVR and switched off fiber router
- Moved Wii out of the cabinet as far as the AV cable could get it away

Not sure why it works fine in the menus. Apparently "The Wii MotionPlus uses a Tuning fork gyroscope, which supplements the accelerometer and Sensor Bar capabilities of the Wii Remote" so perhaps that has something to do with it?

We also share a wall (that the TV is up against) in a semi-detached unit, so who knows what our neighbours have set up on the other side in the interim.