score:6
Assuming I've understood that correctly, I think I'd always be fine to go from A to C on one day, then stop off in B on my way home. Is that correct?
Yes. So long as the timings mean that all the trains you travel on are OK with your ticket (ie, you don't stray into any applicable evening peak).
What about a break of journey on the outbound though. Is it commonly allowed, or commonly restricted?
It... depends :)
And how do I find out for my given journey?
The information is there on the National Rail website, but as far as I can tell there isn't any well-publicised link in. The actual PDF you that has all the details is (currently) at http://www.nationalrail.co.uk/pdfs/OffPeak.pdf, but it's best to note how I got there for when they inevitably move it:
Other tickets β
section1 x Adult - _Off Peak Return_
http://ojp.nationalrail.co.uk/service/ticketterms/show
and contains information along the lines of ticketTypeCode=SVR&ticketRestriction=US
- this US
is the Validity Code within the ticket type of SVR
(Saver Return, the old name for a multi-day off peak return)US
, at http://www.nationalrail.co.uk/pdfs/SVR_US.pdf, and information for all offpeak validity codes, at http://www.nationalrail.co.uk/pdfs/OffPeak.pdfOnce you have this last PDF (and the similar ones for Advance and Anytime), you can just mouseover the links on the journey planner results screen and look up the codes yourself.