With the record from 1986, the highest March trmp. here in Wembury was 19..5°C on 29th in 2012.
However, not far away at Plymouth Mt Batten it reached 20.1°C.

Len
Wembury

18.9C in Penzance on that date. Since then the highest 17.7C on 25/3/17

MetO are going for 15C for Penzance on Tuesday, however that goes with a forecast 5mph onshore SE wind. Assuming the north coast sea breeze develops, which seems likely, that often backs the wind in Penzance to an NE offshore by mid afternoon. (The NW facing north coast sea breeze is stronger in the afternoon). If that happens then it'll make quite a difference, 19C currently forecast for Bude.

Graham
Penzance