I want to make a button for changing a selection to checked tick boxes and vice versa.
I've looked through the edit menu to customize it but can't fin the commands or arguments anywhere. It must be me but what am i missing?
I don't know, if this also solves the "vice versa" part, but it is a good point to start with:
@ifset: CHECKBOXMODE=Toggle
Select FROMCHECKS
Set CHECKBOXMODE=Off
@ifset:else
Select TOCHECKS
Properties SETLABEL=Rot
Oops, sorry, but you have to remove the last line, "Properties".
It's under Edit > Select Other, if you're using the default Opus 11 toolbars/menus.
Otherwise, the commands are Select FROMCHECKS and Select TOCHECKS as used in Abr's button above. (Abr's button will also toggle you in and out of checkbox mode, if you want that as well, all on a single button.)
As ever, I am in your debt - thanks gentlemen.
please dumb down the how2 add a button to menu that would perform a "checkboxes to selection" action
There are buttons already made for you in the default menus these days, under Edit > Select Other.
(If you need help creating buttons in general, see the FAQs, or the pinned post at the top of the Buttons/Scripts area.)
Yup I used those frequently during the day. Hoping to cut down mouse clicks and move to main task bar or right click menu.
Settings / Customize Toolbars...
Then right click the menu item you want and paste it to a toolbar or menu wherever you'd like.
As Leo said, see the FAQs, or the pinned post at the top of the Buttons/Scripts area.
Much closer but i don't see an item: edit/select other/checkboxes to selection
You said you used the item regularly....
Read the FAQs Leo and I mentioned they will show you how to create a toolbar/menu button. Then with that knowledge you can create a button with the commands in the previous post by abr.
If the menu items aren't there then you might be using toolbars from an older version of Opus.
For the standard Menu and Operations toolbars, you can reset them to the current "factory defaults" via the menu you get by right-clicking an empty part of the toolbar:
If you're using completely custom toolbars (not old or modified versions of the default toolbars), then that menu item won't appear, but you can reset all toolbars back to the defaults via Toolbars > Factory Reset Toolbars in the same menu.
If you don't want to reset them and just want to add the commands from earlier in the thread manually, see the guide:
I reset the toolbar but I don't think that changed anything.
Just seems that the command i want, "checkboxes to selection" is not available via the customize menu.
which faq would explain how to add a button to the toolbar for something like that?
If "Customize menu" means the list of pre-made buttons you can drag out of the Customize window's Commands tab, it is not in that list. (That list only has some simple things in it. Opus commands have potentially millions of variants, so there isn't an exhaustive list of all of them.)
But you can find the command under Edit > Select Other, in the default toolbars. And you can copy that to anywhere you want on your toolbars.
While editing toolbars (which you start via Settings > Customize Toolbars), you can copy and paste items by right-clicking them, if you want to duplicate or move them around, e.g. to give yourself a top-level toolbar button for something that was in a menu.
You can also create buttons and menu items from scratch if there isn't already one to do what you want.
The one we keep asking you to read. 
https://www.gpsoft.com.au/help/opus12/index.html#!Documents/Creating_your_own_buttons.htm
and heres a check to selection toggle button, courtesy @abr, many thanks.
Checks.dcf (549 Bytes)
right-click on a toolbar and customize then drag this file from where you have it downloaded to the toolbar you want.