St. Swithin's Day - According to weather folklore will it rain for the next 40 days?

The 15th July marks St Swithins day which is of "some" importance in the weather calendar

Legend has it that if it rains on St Swithin's day it will rain for the next 40 days, but if it's sunny on St Swithin's day it will be sunny for the next 40 days! But is it true? Good Morning Britain meteorologist Laura Tobin explains whether there's any truth behind this weather folklore...