Thanks for your help Andrei, much appreciated. 
)
Actually, passing the challenge was easy, after I realised +I+ was supposed to make the challenge directory in my own public directory (I’d been waiting, assuming Pair had to put that somewhere I don’t have access.)
Perhaps I also misunderstood about Certbot too, I’ll check again if I can install that in my own shared web space at Pair, but the Cerbot webpage is daunting to non-geeks like me and your Zerossl.com suggestion was what got me along the way.