I share your pain
Yes, you cant even put a <CR> into the box where you add more information which makes it close to impossible to write anything even remotely readable when it is more than a few word in length.
And as for prohibiting the percent sign! Words fail me - it is a fscking finance site!
I have several times put a complaint into the feedback link - never got a response either.
I had a huge problem signing up on the HMRC site in the first place as I was in Qatar. The password mail took 3-4 weeks to arrive but was only valid for 2 weeks. When I called them up to ask what I was supposed to do they suggested I got it sent to someone in the UK who could phone it to me. I think they have completely lost the plot. After all, what is the point of insisting they send out a super secret code then because they fscked up the expiry telling people to send it to someone else!