professor Crystal is but one source amongst many..that does not make him right as you would see if you checked the British Library link and the one on the accents and dialects thread.
I grew up with Devonshire dialect....left it behind but still have an almost hardly noticeable Devonshire accent.
Perhaps if people actually looked at the Dictionary definitions of accent and dialect...they might be a little more understanding of the whole silly debate.
There are many dialects and many accents and I don't have to be a professor to understand that..I only have to read what many others have written on the subjects.