Thanks, Dean, that has clarified a few things. After reading the manual quite a few times, I was coming to the conclusion that G28 was effectively "move to machine zero", but with an optional intermediate point. Similar to Mach3 and MyCNC except that these allow a G28 position definition rather than assuming machine zero. I'm used to seeing code that has something like:
G28 G91 Z0
G90
near the end - meaning raise to machine Z0 before going to ref posn in XY, so you don't clout anything on the way. What I'm not sure about (because the manual isn't that easy to understand) is whether this sequence would work on the SZGH controller as there is something about not moving along an axis if it is not specified in the G28, so you would actually need something like:
G28 G91 X Y Z0
G90
- but maybe not!
Looking at G30, I get the impression that it does exactly the same as G28 except that it uses the current work coord zero. But I'm guessing a bit here as the manual is not at all clear. In particular, it describes G28 in detail, then mentions G281/282/283/284 which do the same thing but only in X/Y/Z/A axis. It also talks about G301/302/303/304 which are similar but G30 is not mentioned in this section at all - although it is in the table of recognised gcodes.

So, in practice, on this controller, if you have a fixed position tool setter or tool change position, would you arrange for this to be at machine coord zero, and arrange home switches at limits of travel but with appropriate offsets for the home position? Then G28 would work in the way that we currently know and love (including use of G91/90 as in my code snippet above).