Thanks for your kind efforts, everyone.
I am currently using Ancestry UK, so have the later information, although the records for a John Levi Legg (with a daughter Mary Ann) muddy the waters.
However, he's definitely listed in other census documents as having been born in St Peter Port, and the surname is indeed common on the Channel Islands, so unless he falsified his entries I'm still hoping to find his parents on Guernsey.
The Devon connection: yes, he married Elizabeth Miffling at Exeter and in 1861 they were living in Southampton, with her recorded as having been born in 'Tattness, Devon' (Totnes).
So, back on the Guernsey trail... :0)