Principal Designer?

So our architect was extremely unhelpful on our self build; as soon as he had the full payment, he was entirely disinterested. Whenever a problem cropped up he didn't want to liaise with anyone or help with a solution; this was left to us and the builder. Builder doesn't understand why architect became basically combative. Suffice to say, we have not had him on the build for a long while. Architect never resigned as principal designer or alluded to resigning. He is on paper as our PD. So - now he has nothing to do with the build, who is the PD? Do we find a new architect and ask them to be PD? I've read the principal contractor becomes PD in a case like this; the builder is PC and has overseen that it's being built to design and has taken all the photos etc of every stage. However, I've also read on the Building Control site (Stroma) that we as the self builders can sign off as PD.

Help. I don't want to break any rule or make a huge mistake. Any advice? There have been no changes from the main design, it's been built entirely to the correct spec etc. We may have a staircase tweak as the architect said stairs would be designed by stair company, and the stairs planned by the architect do not work to regs.