I've got a couple kicking around - an X9 (short), X0 (long cage) and an X7 (long cage) - you can try one if you like.
Internet seems to think capacity of a short cage SRAM MTB is 32 so with a 53/39 12-30 you would have a set-up that on paper would work if you went short. Unless I can't math.
@Howard and @edscoble and @mdcc_tester
Excellent news. Will try fiddling with B screw but otherwise will search for cheap MTB mech. Thanks all.