So the advert says "you can do your tax return at hmrc.gov.uk" and even shows "hmrc.gov.uk" on the screen.
hmrc.gov.uk does not resolve - no A record (or AAAA record).
Seems www.hrmc.gov.uk does have an A record though.
How hard is it to make a web site work without the www on the front? Clueless or what.
(and before anyone says, no, www.me.uk can't work without the www. It's one of the few exceptions).