Hi All
Final update. I changed the points which solved that issue and then addressed the second radius curve. By checking the level of the track and adjusting I managed to get the loco to negotiate the curve. The tolerances here are small - my spirit level bubble is in the centre, just a smidge off and the loco behaves (just). It always feels that it is barely clinging on and as mentioned before you can’t take your eyes off it for a second. As a sanity check I ran my HST “Farewell Tour” set over the same track with absolutely no dramas.
I then moved on to the third radius and the loco was straight off on the first curve. As JSpencer mentioned there is no lateral movement on the front bogie so the slightest undulation on a curve and the front bogie lifts throwing the loco off the track every time.
Refund time and order placed for Class 87 instead.