Arghhh - can't get used to this
Thankfully its only temporary. Took the day off work today and went to war against the duckweed. Took out the frogbit and threw about 80% of it out leaving only small new plants and single leaves. Washed them thoroughly (several times) and left them in a bucket for several hours. Dropped the water level by a couple of inches to ensure nothing could get caught in the top of the tank. Netted out everything I could find.
Then I put a nano powerhead in at one end (back right) and a homemade surface skimmer at the other (front left) and left this running for a couple of hours before putting the remnants of the frogbit back. Of course with the new flow, and nothing to restrict the frogbit this means the light and dark sides of the tank have switched. I trimmed the vals so they no longer wrapped over the top and added an extra bag of leaves to reduce the light, hope the anubias aren't shocked.
I'll leave the powerhead and skimmer running for a while. Hopefully the reduced mass of frogbit will allow any remaining duckweed to filter through to the skimmer, especially if I stir up the surface regularly.