why was the groom polishing the silver in the dining room?
I just can't make out at all what job Mr Bidwell is supposed to have. It's implied that he was head coachman but no longer is; but what is he now? He wears un-smart outdoor clothes which would be appropriate to maybe a groom or a gardener; certainly not to a butler, who would be the man responsible for the silver. (And, of course, an ex-coachman simply wouldn't be qualified to become the butler - totally different skill set.)
And whatever he was, why the hell was he polishing the silver in the dining room? You don't need to have heard of butlers' pantries to know that of all the places where it isn't a good idea to spread out rags and polish and get rubbing, the dining-room fully dressed for the poshest dinner of the year is a front runner. I can only suppose that they'd put so much energy into dressing the kitchen and the dining room that when the director called for a scene of Bidwell glumly polishing the silver they said 'Oh hell, we forgot to do a pantry set-up - never mind, he can be doing that in the dining room'.
That scene actually gave me a spasm of reminiscent embarrassment; I used to belong to a semi-professional group offering costumed historical interpretation. We had regular gigs at several historic houses, but occasionally we'd get a call from a new site. Our organiser would ask 'Is it just a party of gentry and a footman and maid or two that you want, or do you want a below-stairs presentation as well?' A couple of times the client said 'Ooh, an upstairs-downstairs angle would really add value, yes please', so some of us turned up in servant outfits with aprons, facsimile recipe books, utensils and tools - only to find that there were no below-stairs rooms for us to use, and the client had idiotically supposed we could 'be servants working' in the same space as 'the gentry taking tea'. And we spent the day on the landing or the corridor, or once even huddled at the other end of the drawing room, trying to find some semblance of an excuse for being there and hoping we didn't look as stupid as we felt. Maybe I'm wrong but I thought the actor looked a bit like that!