Cheers, really helpful. Just to be clear, with those 9sp derailleurs, I'll still be able to use a 10 speed not a 9 speed cassette right? Maybe I should just use the 105 5700 medium cage which apparently works up to 32t and will presumably play perfectly with my shifters.
Yes. It's the shifters and the cassette that 'have' to match as they decide the movement. The rear mech just needs to be able to move the necessary amount.
This will work up to 36t if you like;
https://www.bike24.com/1.php?content=8;product=97200;menu=1000,2,111;page=7
another example (up to 34t).
https://www.bike24.com/1.php?content=8;product=63250;menu=1000,2,111;page=8