Oh crumbs. Here we go!

[Jonathan]

Do you know if that applies to the sides too?

It would certainly help, but even better would be to lower the gantry as much as you can and have the tool operating below the level of the X-rails.

Was just about to order some aluminium plate to start building my z axis. Had a bit of a panic. Is 15mm thick enough or would I be better off using 20mm for the sliding part and the back plate?

15mm is OK, but most use 20mm as you don't gain much by going above 20mm. In the whole scheme of things it's not going to cost too much for the extra 5mm...

In answer to an earlier question - standard 6082 is fine for supported rails. Tooling plate is only really worthwhile for proper linear guides.