Argument over changes to attendance times over crimbo

[Mezza]

I approached my DOM on Friday last week and asked if i was going to change my times of attendance over Christmas (i did it last year and it worked to the benefit of all parties).
I work RLE but assist the callers with booking in items at end of day.

With the expected overtime etc we found last year that we had loads of stuff coming into the callers the nest morning and the single callers office duty couldn't cope - so i changed my hours to start and finish an hour earlier. This worked as there would have been 3 of us in the callers for the last hour of each day with just the 2 computers to use (meaning 1 person was surplus more often than not).
So having an extra person to book things in in the morning made good sense and proved to be a good idea.

My DOM agreed to the change on Friday but this year there is an issue -
On Monday and Tuesday the person that is due to cover the callers from 12pm till close is our Union Rep. He often covers the callers late on Wednesdays etc - but is never, ever back from delivery in time to start his shift in the callers.
His official duty end time on Monday and Tuesday is 14.03 - so i'm not sure how anybody thought he would be able to do the callers from 12. He has never made it back to cover the callers on a Wednesday at 14.30.
And so it has been proved - yesterday there was no sign of him at 13.00 (let alone 12.00) and that was the time both the regular callers duty and my own finished and we went home.
The Line manager (Yesterday was DOMS day off) said he knew nothing of who was covering the callers and as he wasn't going to cover it he locked the door (we were due to close at 19.30).

The impact of that was a mountain of stuff to book in this morning (nothing had been booked in yesterday and complaints all day from customers who tried to collect items yesterday.

The DOMs solution is to now revert back to my usual times of attendance on Monday and Tuesday but start and finish and hour earlier for the rest of the week as we agreed.

I feel this is grossly unfair as it will mean i now have to cover the callers duty for the last hour Monday and Tuesday(not just booking in items as i normally would , but having to serve) - nothing will get booked in unless the Union Rep decides to when he finally arrives. I have to mess about with my start times and all to keep the Union rep happy with his SA.

I should also point out that the DOM says nobody else wants to cover the callers despite the fact that absolutely nobody else has been asked.

Can my DOM just chop and change my hours like this?

He's given me the option of simply going back to my regular hours all week - but that would still mean i'm left to man the callers office single handed for an hour to 90 mins a day every day and the whole reason for the change in the first place was to assist my colleague in the callers at the worst time of day - ie first thing when there's loads to book in and she has to open the doors 30 mins earlier over xmas.


Any advice would be welcome.