It depends on the boots in my experience. I bought some one year that fitted perfectly then ordered exactly the same size the following year and they were far too tight on the foot. I stupidly kept them thinking it was just because they were new and they would wear in but they never did. If you're anywhere near London they have a shop where you can go and try them on.
I forgot to say I phoned the Edinburgh shop before purchasing online and took their advice to size down. When I was in the shop at a later date tried on the Brindisi in my true size and they were like boats!