Where can I find the restrictions and easements that apply to a given UK Rail Ticket?

UK rail ticketing isn't always the simplest thing in the world, but the basics aren't too hard to learn. There are broadly tickets valid at any time of the day, those available only outside of peak hours (typically after 9 and perhaps not in the evening rush hour), tickets only available at quiet times, tickets valid only on one train. These are mostly, but not always, valid in flavours of single (one way), day return (out and back the same day-ish), and (period) return (out on a given day, back within a month). That's it broadly, but you'll notice there's a bit of variation in even just those...

Next up, there are rules on what sort of route you can take. These largely revolve around routing points, not doubling back, direct trains normally being fine, and not taking too long a route if there's a choice. (There's a whole art to making the most of that though, but this isn't the question for that!)

On top of all that, certain routes and/or stations will have additional restrictions, or easements where default restrictions are lifted. For example, one station might be in an area where the default rule is "off peak tickets valid after 9am", then has an additional restriction "off peak tickets to London only valid on trains arriving after 10am", and an easement "off peak tickets to stations along the insert line name here valid after 08:45".

By looking up these restrictions and easements, it's sometimes possible to save money on your journey. In the example above, you might assume you'd need a peak time ticket to travel on the 08:54 to a station along the local branch line, but the easement actually allows you to travel on a cheaper off-peak ticket instead.

How do you go about looking up what easements apply to a given ticket for a journey, and equally what extra restrictions (eg evening peak times or not-valid-via) apply?