Never heard of Thomas Guest concertinas. Were/are they any good?

I very much suspect that neither has anybody else !

I assume that you are referring to the example for sale on eBay, which looks very much like it was made by George Jones. In which case it may have either brass, or maybe Jones' "broad steel reeds", and a mellow sound, but don't expect a fast response.

It was normal practice for nineteenth century manufacturers to make "own brand" instruments for dealers. Both Lachenal and Jones did it a lot.

I have done a little genealogical research on Thomas Guest of Exeter this afternoon:

Thomas Squance Guest was born in Exeter on 16th May 1823, and was Christened a Wesleyan Methodist. An 1850 dirctory lists a Thomas Guest, Siversmith & Jeweller, 35, New Bridge Street, but by 1857 the entry is for T.S. Guest, Jeweller & Dealer in Musical Instruments, at the same address. On the 1881 Census he is a Music Seller, and on that for 1891 he is a Music Seller & Repairer (employer). An 1895 directory lists T.S. Guest as Musical Instrument Repairers at 35, New Bridge Street, as well as T. Guest, Pianoforte & Music Warehouse, at 40, New Bridge Street. He died at Exeter in the third quarter of 1895, aged 72.

(The label on the eBay concertina reads Thos. S. Guest, New Bridge Street, Exeter, but the street number is hidden by the fretwork.)

When repairing concertinas, and other antique musical instruments myself, I have sometimes had to call on the services of a silversmith, so I find it easy to see a link between the two trades. It was the jeweller/clock repairer in Kilrush who used to fix the reeds in Mrs. Crotty's concertina !