Re: If this little "feature" is still open to abuse?
@Martin: Starting with a clean copy of Firefox portable, which did let me in, then loading addons one or two at a time, Ghostery and scripts from Google Analytics had been the only things I could see that were left to test before I gave up trying to find out what was blocking my access to the HMRC self assessment site. (Life is short, after all.)
Today I seem to be able to get in without any problem, and apparently without a Flash Cookie being planted; though I didn't enter any data. Ghostery showed 0 trackers on the self-assessment menu page. Maybe government techies do read El Reg after all, but I still haven't had an offer of employment.