looking at the brakes I’m taking off – Hayes – am I right in thinking they are exactly the same, front and rear ?
If so, does that apply to every make ?
Obviously the hose is shorter but a lot of levers are flippers now.
Mine are international Standard if that makes any difference…

yes

If IS then then on the front its a 20 mm bigger disc than it would be on the back

Mine are both 160’s so I guess if the new set are the same rotor size (ofcourse) it would work ? Want to keep my rotors as they are really light, you see..

Not quite the offset of disc brake calipers is slightly different back to front, you can do it but you will need extra spacers and possibly longer bolts to compensate and as TJ says above then you will need a smaller disc for the back.

Are the brakes actually IS only. Quite a lot of them are post mount with IS adapters. If that is the case then you can swap the adapters and use the same disks.

You can swap any brakes if you have enough hose and an American to ride the bike for you.