Thanks for the advice; apart from anything else, I now know how to check the md5 sum. Success with the flashing, though; humble apologies, I wasn't in fastboot - it was highlighted in the hboot menu, but I hadn't selected it. Once I did, fastboot commander worked fine, & after restoring the backup I've now got 121Mb used in internal storage . . . and 215Mb free. Stuck with the oxygen r2 in the end, but if I do catch the playing with ROMs disease, I'll follow your recommendation of cyanogenmod.
& if you're in North Shropshire in the next month or so, there's some bundles of asparagus with your name on . . .