Help with rolling contract being terminated/union issue

[R1gtr]

Good Afternoon,
Hoping that someone can maybe help with some advice on how best to proceed or if there is maybe no point in wasting my time.

Will try and keep this as short as possible but I am known to waffle so bear with me.

I have worked for a Delivery Company for 20 years delivering letters and parcels, I am contracted to do 5 days a week and we typically work 6-2 Mon-Sat.

We are a small office and 6 years ago we had our Collections driver retire. This was a 15:30-19:00 role Mon-Fri and myself and a couple of others took this role on to help until they found someone to replace the retired member of staff..
This continued for a few years informally with no issues but then our Manager was trying to save/move hours and wanted to pass on the collections to a larger distribution office.
We contacted our area union rep who was fantastic and had that shut down and said that the Collections was ours and even had a rolling 12 month contract called 'Scheduled Attendance' drawn up for us. It does state that it can be terminated by the Area manager with a month's notice if he has good reason.

This brings us to the current day 6 or 7 years down the line of covering this every week without fail and zero reliability issues. Myself and the other 4 members cover for each other if someone is sick etc.
However relationship with the union in the office has got really bad and a vote of No Confidence letter on the office rep was signed by most of the staff and sent to the union, nothing serious, just not fulfilling his role and being uncommunicative and unapproachable, and ever since then the rep has been trying to make life difficult for everyone who signed the letter.

This has resulted in the rep signing an agreement with the Area manager saying that the Collection duty should be advertised immediately for permanent filling. He argues that it should have been advertised 6/7 years ago. We have been told that we can apply for it but it would be in place of our morning/ full time job.

We feel that this is coming from our rep and not the Area Manager, and that this is punishment for speaking out. There have been other instances of petty revenge but not relevant to this issue.

I am wondering if we have any protection/ right to the role after doing it for so long? Etc
Any suggestions on how to resolve this?

Normally we would go to the union for help on issues but this does not seem like an option as I think there is a conflict of interest. We have a new member of staff who has had a lot of issues with sickness/childcare arrangements and our previous manager took her on from another office where they were having issues. The Union helped this member negotiate a Mon-Fri contract whilst everyone else has to work Saturdays. This member was heard early on saying the Collection duty that we do would be perfect.

It feels like this is the number one reason the collection duty is being taken away from us, to find a role for a problem member of staff.

Should we just admit defeat or is there any grounds to appeal/fight it?

Thanks for reading, any suggestions welcome.