Mrs Gertrude Roberts perhaps was the Mrs Gertrude Roberts who was living in 1920 at Aubrey Lodge, Aubrey House, Kensington, by Campden Hill Square. The House had an early history of suffrage activity, from the 1860s and in November 1933 the Lodge was advertised as being rentable as flats. During WW1 it was a hospital, probably closing in 1920. Mary Stocks retired there in 1915, so it has a good “pedigree”. 
Can you add more to our brief pen portrait of Gertrude and/or have ideas for other candidates for her seat at the table? Gertrude at Aubrey Lodge could well be the one, but our evidence is very thin. Table 22 has such a strong suffrage flavour that our Aubrey Lodge candidate is ideal!
There are other Mrs Gertrude Roberts in the electoral registers in London in the 1930s but so far none suggest any relevant connection to the dinner.
 Various references to the Lodge in The Times including: 3.7.1919 and 2.11.1933 and the 1920 PO Directory records Mrs Gertrude Roberts living at Aubrey Lodge.