I can't even use the online filing...
... because their systems are Microsoft systems that are horribly broken, and don't comply with RFC2822 - so as the local part of my e-mail address as used for HMR&C correspondence is, yes, you've guessed "hmr&c" (perfectly legal as defined by RFC2822 but declare an "illegal e-mail address" by Microsoft's electric Meccano), I can't register or log in.
Of course, the HMR&C tech support droids couldn't appreciate the irony of the long exchange of e-mails about this issue to exactly that e-mail address, with (of course) no difficulties on either side whatsoever... :-(
Their position is that "this is as specified in the GovTalk standard in the UK", GovTalk being not a "standard" at all, but a Microsoft specification for public sector systems to protect their amateurishly-written operating systems, to allow for the gaping security vulnerabilities in Windows arising from use of the ampersand (&) character.
I don't see the issue, Microsoft could just produce a (much more secure and reliable) Linux-hosted system... :-)