"They have had a bitter quarrel I think" said he and bit into the bagel. "They're rummuns" said he chewing it.
This could easily be Norfolk dialect, though the word rummun has clearly been part of general British speech. It is nowadays a bit dated.
It means 'rum one', the adjective rum meaning strange, odd, different, peculiar etc.
A Norfolk expression Tha's a rummun ent it means 'It's a strange thing, isn't it'.
*They're rummuns' would mean simply 'they are odd/strange ones'. It can equally be applied to people as to objects etc.
Rum used as an adjective in this way is confirmed by the OED (sense 6) as dated, and they are uncertain of its etymology. It is predominantly British, but I wouldn't mind betting that many older-generation Australians, New Zealanders etc use it. A few of the more recent examples from the OED:
1955 J. Thomas No Banners vii. 61 ‘This is a rum go,’ Alfred said.
1977 J. I. M. Stewart Madonna of Astrolabe xi. 153 Some Scottish names are distinctly rum. Yours is.
1993 A. Habens in M. Bradbury & A. Motion New Writing 2 247 It's a rum do if a chap isn't allowed to remember what he remembers.
The adjective rum gives rise to may composites e.g. rum-looking, rum-sounding etc