After 2+ weeks resolved the issue with help from Emma_C.
After trying this and that with the 2nd SIM, Emma_C arranged for me to get yet another SIM and she ensured that it was enabled. This meant that the issue was somewhere else.
In desperation I tried my wife's sim in my phone - it didn't work, then I tried the new shiny Sim in my wife's and it made contact with the network. Switched the Sims back and both phones were still working.
For some reason my phone had become 'locked', maybe when my old sim was disabled.