Thanks Nic. I'm glad i'm not going mad because I was doing exactly that; typing "2" as the regex for the select list.
I've done loads of testing over the weekend and what I found out is that if I create a section and add the option list question first and then 3 text questions, it doesn't work. Matrix continues to tell me to the option question is a required field. If however I create the text questions first and then the option list question, add the validation and test it does work. Maybe there's a bug or something up with my install?
I'm running 5.1.3 so upgrading may fix that issue. Either way I've worked out how to get around it for now which fixes my issue.
Thanks for all your help over the last few days.