jonathan18: Umm, maybe leaving it until 29 hours before D-day to buy RUCs for both our cars was a dumb idea! Spinning wheel of death on the website at moment; will try again later tonight and, if no luck then, first thing tomorrow.
I continued to have trouble all the additional times I attempted last night, and again early this morning (before 6, on both iPad and laptop) - it either got stuck on the payment screen, or the WK website threw up various errors (including sometimes telling me I had to call them, or that I couldn't buy RUCs for a specific vehicle).
But tried again about 8sh (on laptop) and it went through without issue.