A question that I am (sort of) qualified to reply to as I also have a large skull. The answer, like most things, is "It depends..." I have a Giro something-or-other. One of the more expensive models as they come in various sizes rather than the cheaper ones which are one size fits all (they don't). BUT it does depend on the shape of your skull. Mine seems to be oval shaped rather than round so the Giro one fits well, but the large Specialised one is tight at the front but has loads of room at the sides so is more suitable for people with "rounder" skulls. I have also found that Bell helmets fit me too. NB: I bought the Bell helmet at least 10 years ago so the shape of them may well have changed, and the current Giro one is 5 years old now so may be an old model. When I bought the Giro I also found that Met helmets didn't fit, being a similar shape to the Specialised one. You do really need to try them on rather than going for mail order, but appreciate that that might not be so easy as I recollect that you live in Snowdonia. It is worth persevering and getting one that fits well as mine is so comfortable that I am not really aware that it is on my head.