Update!!
The bump steer has gone completely! I was faffing with stiffening the front suspension to prevent the bump steer but that was just restricting the amount of travel, rather than fixing the problem. What I wanted to do, was drop the steering rack so the track rods were parallel with the suspension arms. That wasn't possible as there were pedals in the way. So I fitted the track rod ends into the top of the steering arms. BINGO!
Moving the suspension from top to bottom and there's no measurable bump movement!
I tapered the arms to suit the track rod end tapers which obviously didn't make a full taper as it met the original taper going the other way (which irked my engineering head) but it all nipped up tight.
Yesterdays trackday confirmed that the handling woes have gone. I need to soften the front end a bit and reduce the toe-out slightly ready for the next trackday in July.
https://www.youtube.com/watch?v=77rr_qvDIDE