Hmm! All the swatches I've done so far have been at least 25 stitches across and maybe 20 rows down, to try and get a decent middle section! I think I'm going to just risk it on the 5.5, as foolish as it may be! At least then I know I'm only getting 1 extra stitch per inch.. I'm planning to line it with fleece after (to keep out the cold cornish wind!) so if it is a little big, that should help? We'll see! If nothing else, it will be an adventure!
Thanks very much for all your help! It was very very much appreciated.