I have written to the club already. Hopefully they resolve the issue.
I don't think I've ever tried to buy tickets from my work computer so having used a VPN can't have been the problem. (Although I may have generally logged into the website to read a news article etc.)
Evebtually, I logged in today using my mobile network and managed to get 3 tickets together. Might have been the first to get in tbh - 10 seconds past 10 a.m. I've been pretty lucky overall with tickets. Apart from Spurs I've managed to bag tickets to all the games I've wanted to. Have the next four after City done as well. Hope I'm able to get the tickets for the last four home games too. Afterall, I don't want to miss us lifting the trophy in May 😉