Thank you to everyone for replies. I'm still wading through them but trying to avoid spending time on the computer.
I survived the weekend. Saturday night was so bad that the only thing that stopped me from ringing 999 overnight was the fear that I would be told there were no ambulances available. I simply stopped trying to do anything except rest and keep my heart rate from spiking again.
First thing on Monday I was no.16 in the queue to speak to my GP (e-consult wouldn't accept my request, advising me that I needed to speak to a live person -- yes please!!!!). Got an emergency appointment. I had the good luck to see the GP I wanted to see, one who listens properly and doesn't waste time on blaming (why did/didn't I...). He confirmed that I did the right thing in not taking the flecainide because it's an old prescription + I had no idea how long I had been in arrhythmia before the tachycardia episode, but he was not prepared to issue any new prescriptions without input from a cardiologist. He got straight on the phone to the cardiology dept. at the large teaching hospital 20 miles away, which is my preferred place to go (our small local hospital does not have a good reputation, alas).
So, the head of cardio is due to ring me direct -- within the next day or two -- to see how I am and talk through new prescriptions, which will probably involve an anticoagulant at least temporarily. He may want me to go in for cardioversion, depending on whether I'm still in fib.
In the meantime I'm managing with metoprolol twice a day as a baseline, to be increased if needed. I'm taking a conservative approach because I've rarely taken them, so still on 12.5mg, which is keeping me at 65--75bpm at first, but the effect tapers off within four or five hours and I end up in the 80s. The GP agreed that 12.5mg is one step up from a placebo and told me I can take 50mg 4x/day. If my heart rate goes over 100 -- raise the dose. If I raise the dose and I still go over 100 -- call an ambulance. Crystal clear advice.
I finally slept normally last night (the seventh night), THANK GOD, and feel more like myself today, although still in fib.
The GP also confirmed that there were no cardiac markers in my bloodwork from A&E (which no one told me at the time; I inferred it from the fact that A&E sent me home), so that was good news.
Big lesson: I should have phoned the GP in the first instance, but on previous form I thought they would send me straight to A&E and I decided to cut out waiting time by ringing 999, which proved to be a big mistake.
Another one: I need to upgrade my technology so that I can monitor myself effectively. Most of my AF is slow AF (normal or only slightly elevated heart rate). It may have been going on for a lot longer than I thought.
I've postponed the Covid booster I was due to get tomorrow.
Thank you all again for the input. Stay safe.