someone posted about Ribbles cheap slx brakes the other day and as i've got mrs antigee's bike in bits and discovered i'd pilfered for my daughter the brakes i was going to fit…

question forks am fitting are rigid carbon with IS mount

says on Ribble site "Post mount with 160 IS adapter"
does that mean will run with a 160mm rotor?

seem to recall something about adding 20mm because of the adapter (adaptor?) or am i confusing with PM to IS

It's a post mount calliper, and comes with an adaptor to run with a 160 disc on an IS fork. If you had a PM fork it would bolt straight on, but looks like you'l need it,

APF

thanks alexpalacefan – that was how i read it and that is what i want to do
think getting myself confused with putting an IS caliper on a PM fork think that when need to increase rotor size

Yup, if they're the same as the XTs from Ribble, both ends come with IS to post adapters suitable for 160 rotors. I wanted 180 front (reusing my old Magura rotors) so had to buy a +20mm front mount.