It stuck me as odd that I not infrequently encounter inter-unit doors locked when the trains are mostly 8 car all day. You've hit on the reason - helps the conductor to have a quiet life.
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As far as announcements go, usually nothing when train is delayed.
In my experience, LNwR conductors often walk up and down the train without checking tickets. Can't understand why, unless it's to appear on the CCTV.
But very recently I've seen groups of three staff inspecting tickets and issuing excesses, not the conductor, but dressed in a black LNwR uniform. They seem to find a lot of passengers without a valid ticket.
But, to be fair, there are a few conductors who do a thorough job (check tickets, make useful announcements) - the same few!
There can be legitimate reasons for the gang ways being closed, often when having a ride out it is done as you can't fit more than 1 person in the cab when doors are in the open gangway position. The only other time to lock out the gangway is for extra space when cashing up. All other times it is expected to be available to passengers.
Revenue is very depot specific, you will find less staff doing revenue in the evenings on certain routes due to a broken safe at one depot meaning machines have to be back prior to 10pm (or the break prior) due to cash regulations.
There has been more RPIs recruited, and the company is pushing this side hard. As a minimum you should see a conductor scanning once per journey, so it's only if you do the full trip that you should definitely be checked.
As for walking through but not selling tickets, again as per above. Guards may not have a machine (or a faulty one), so the emphasis is then on being visible if possible and checking the train for antisocial behavior, train faults etc.
There was a new guide for announcements this year which the first point on the 'Do Not' page was 'duplicate automated announcements' so other than mandatory, UDS and delay announcements we are meant to leave it to the train to do.