A very interesting can of worms, Mick. It introduces the idea of the condition rarity - coins that are common in low grade, but disproportionately scarce in high grades. The best insight, over and above my own findings (which, after all this time, are not inconsiderable), I've found is the book "the Standard Catalogue of English Milled Coinage", written by Cope and Rayner, published 1975, in which they give relative rarity ratings for coins in a range of grades.
Rob Pearce sold me my copy, he may have some more, but it's an excellent reference...