Having heard nothing about my complaint after a week and still no access to my account, I called Natwest helpline on Thursday evening and waited 20 minutes only to be told they couldn't help/didn't know what's happened and could only suggest that I try visiting a branch instead. I'm not impressed.
I nevertheless received a written response to my complaint yesterday. They've rejected it, saying that my account has been blocked by fraud prevention measures and since Natwest fraud prevention procedures were apparently followed correctly, they consider that Natwest has done nothing wrong.
So, it seems that during the account switch process they think it's okay to send me progress reports by text message pretty much every day, or even multiple times per day, yet apparently when they suspect fraud on my account it's acceptable Natwest procedure not to actually contact the account holder at all, by any means, over a period of more than two weeks!
I shall be glad to be rid of this lot again when the time comes!