Hi Fiona
This looks like a very interesting family. I used Brian Lawson's census indexes to track them in all the censuses 1841-1871, and in the Familysearch 1881 index there's no sign of them., nor in BL's 1891.
In 1871, John was 64 and Mary was 57. I ventured to look at the 1871 Ancestry page ( I get very very cross using Ancestry at the moment, now that their so called improvements have made searching tedious and frustrating----anyone else feel like this about the new format ?). One of their children was a professor of gymnastics !
Looking at the burial index, I couldn't see any likely burials, but there are 2 possible wills which might be theirs:
LEWIN, John 1877 Onc A 69 0106484
LEWIN, Mary 1879 Onc A 166 0106489
Jean C