Underthesun
I’m going to take a guess, but I believe the only difference between the M975 at £29.99 on CRC and all the others for the older XT at way less, might be the spring is a larger size (the new XT servo brakes have more pad clearance in the caliper), and there appears to be a retaining cotterpin included aswell. A solution for you might be to keep the spring and cotterpin from your old warn pads, buy the cheaper old XT/XTR pads that are way cheaper, throw away the spring from the new pads (check first to see if they are actually different sizes!) reuse your old spring and cotterpin. If there actually isn’t any difference between the spring size of M975 and the older ones then it’s all a big Shimano con (which it obviously is anyway), you can buy cotterpins at B&Q for 10 a penny.
Might be worth consulting your LBS aswell.