Mixed I/O voltages

As a general rule, encoders are 5V, switches and sensors 12 or 24V as higher is better for immunity.
I am aware there are 24V encoders and 5V sensors, I do have plenty of them, but the above should be used as much as possible.
Since you will be using two Mesa boards, you can use one at 5V and the other at 24V, so you can mix and match as you like it.

I have already bough these Optek sensors uk.rs-online.com/web/p/slotted-optical-switches/4550896/ but they only accept 18v

I found a 24v panisonic slot sensor uk.rs-online.com/web/p/photoelectric-sensors/1254517/
Seems a bit pricey, are there any others that people can recommend?

You can find that type of sensors on old photocopy machines, plenty of them inside, or check the 3D printer shops as some of them use and sell them much cheaper.

Ross77 wrote: I have already bough these Optek sensors uk.rs-online.com/web/p/slotted-optical-switches/4550896/ but they only accept 18v

And 18V is for no more than 3 seconds...

But I think you could still use them. Just wire in a 12V linear regulator at the sensor to provide a local 12V supply.