On the National Rail website it says:
if you wish to stop overnight or it is not possible to complete the journey by 0230 on the day following the last day of validity, you may continue your journey the following morning. Travel must resume before 1200, where the time restrictions that apply to the ticket should be observed and no further break of journey is allowed except to change trains.
The way that is worded suggests 'the last day of validity' only applies to the 'it is not possible to complete the journey by 0230 on the day' statement.
Travel, including break of journey, is allowed, subject to the appropriate time restrictions, throughout the validity of the ticket until the expiry date printed on the ticket. The words quoted above extend that validity in some circumstances after the expiry date printed on the ticket. They do not place any additional restrictions on travel on dates up to and including the expiry date printed on the ticket.
it would seem I can finish my ticket at any point until it expires
Agreed.
I am concerned to use the final leg of my ticket from b'ham to London for fear of getting a fine from yet another inspector who does not know the rules.
Don't worry. It is very unlikely that you will be challenged. If a ticket inspector does make a mistake, have your response prepared. Explain politely that your ticket is valid for unlimited break of journey between the "Start date" and the "Valid until" date printed on the ticket. Make sure you know exactly which train (date and time) you used for the first part of your return journey because that information may be encoded on any inspection stamp on the ticket and the inspector may legitimately ask you that to seek to ensure that the ticket is not used more than once over any part of the journey. If the inspector persists, explain politely that you checked meticulously before travel that unlimited break of journey is permitted on both legs during the validity of the ticket and ask the inspector to show you where it is written down that this is not so. If the inspector still persists, ask them to check their facts with the train company's Control. It is extremely unlikely indeed that you will ever reach this point.
If the inspector still demands payment, you have two choices. You can choose to pay up under protest and recover the money charged wrongly from the train company afterwards. If you choose this option, make sure that you find out the inspector's name before parting with any money, and write it down. Alternatively, you can insist that the inspector issues an Irregular Travel Notice, and the matter is then dealt with in correspondence at a later date. Choose the option which will cause you less stress. And if anything like this does happen, for goodness sake come back here for more advice!
Just one final point about the route you are allowed to take. You are travelling on a London - Huddersfield ticket and you must remain on a permitted route for that ticket. A London - Huddersfield ticket is valid to/from Euston, St Pancras, Kings Cross or Liverpool Street. But, having gone to Birmingham, you are now limited to using Euston only. A London - Birmingham ticket would also be valid to/from Marylebone or Paddington. But, if I am reading the Routeing Guide correctly, these options are not available to you with a London - Huddersfield ticket.
