I've found this guidance on an MOT rules website
http://www.motuk.co.uk/manual_620.htm
There's some overriding guidance
"When checking the driver’s seat adjustment it is not necessary to check that the seat can be secured in all possible positions.
In the case of electrically adjusted seats, it is not necessary to check that any ‘memory position’ function is working."
And then these 3 specific tests
1. Method of inspection: Check the driver’s and all passengers’ seats for security.
Reason for rejection: The security of the driver’s or any passenger’s seat is significantly reduced.
"2. Method of inspection: Check that the backrests of all seats (front and rear) can be secured in their normal upright position.
Reason for rejection: seat back that cannot be secured in the upright position.
3. Method of inspection: Check that the driver’s seat position can be adjusted forwards and backwards and secured in the selected positions.
Reason for rejection: The driver’s seat fore and aft adjustment mechanism not functioning as intended.
So I read that together with the IVA manual to mean I can remove the big spring that makes my seat come upright automatically when the lever is pulled so long as the seat still locks when the lever is released. The lever itself has a separate spring that returns it to the rest position so as long as that is retained, I should be ok. I'll get my hacksaw warmed up!
Cammy, I also found some websites showing how people have retro fitted MX-5 seats into MGBs, TR6s etc and they've all gone for some reshaping of the bolt points on the runners but flattening rather than increasing the angle as you've done (but I think how Robin did it). As I need to gain as much drop as possible I think I'll need to go the flatten route and stick my reinforcement under the floor.