The Roman Map of Britain Sillina? Scilly Isles

Corsula (R&C 275) next
Silumnus, var. Silimnus Pliny NH iv 103
Sylinancim insulam, var Sylinam insulam Sulpicius Severus II 51 3-4

Sillanes insulae 925-930
Syllingar 1150
Sully 1193   
Sillinis insulis Camden

    While Corsula is often considered an error for insula¹, consideration should be made of another possibility. Corsula is positioned first on the cosmographer's island list. The listing, which proceeds from the south to the north, would logically begin with the Scilly Isles. 
    If Cor- were considered intrusive², the remaining sequence of  SULA (or SILLA) would have fit into the etymology of Scilly. Mona, the next entry, could have contributed  further to the name's corruption.
    PNRB pp. 457-9 presents a detailed discussion of the name.

1. It could be an assimilation to the name of the Umbrian town of Corsula.
2. A text divider followed by a mark resembling an apostrophe ( /' ) could have provided the initial Cor-.