Update - Back up plan was put into play
I did try to look the answer up myself but was tight on time & couldn't see what I needed to know. Was hoping someone would be able to give a quick definitive answer.
By Dorchester I was riding solo and soaked after 5 hours of heavy rain, so being able make a quick supermarket stop then knowing which station to head to was really helpful.
Typically the rain ceased within minutes of me stopping riding.