Problem. Solved.

I'm still not 100% sure of what cured it/caused it but it was something in the original carb, or because one nut was a bit loose holding down the carb and spacer, or fuel air mix. Probably a combination of them all.
I went for the refurb carb in the end, from ebay, and fitted it this morning. Made sure the nuts were done up super-tight, as one had come loose. Took off the spacer, cleaned it right up, checked it over for cracks and put it back on with new gasket. Flat spot was still there.

Played with the fuel air mix a bit, and suddenly seemed to find a 'sweet spot' at about 2 - 2 1/4 turns out - a tiny bit either way, and there's the flat spot! It really is about tiny movements to find the place that gives no flat spot - turn too far one way or the other and you've missed it!
But there are no grumbles at speed like there was too, no hesitation if you take your foot off the throttle then put it back on... very happy Lina!!
Not sure what speed I took it up to, as, ahem, speedo's not working at mo, but took it for a good run... all good.

Love my car today! LOL Gonna head out in a while, put some banging tunes on the stereo and show those boy racers how it's done (on the corners anyway)... lol

Thanks everyone, for all your help!!

Lina