Thanks for the update.
You will see Ewan tells me (at 35) that the reception with the OEM aerial is not as good as the stubby one.
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Some progress. I’ve bought a standard ND aerial. It’s a bit longer than the NC one, but I wasn’t that fussed about a stubby one, DAB reception with a stubby seems to be inconsistent (at least for some people), and I don’t mind the look of an aerial on the wing away - being longer has improved it in my opinion.
Anyway, I now have good DAB reception on all bands, apart from the one with BBC stations.
This is using the splitter, and remember I’d already modified the antenna base and had the power to it disconnected.
Just got to work out where the BBC stations are now - the car was in the garage at the time so maybe that influenced this??
UPDATE - now able to get all BBC stations 
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Sorry to bring up an old thread ! I have had a Kenwood head unit fitted and I’m trying to get a better signal on DAB - the fitter added an adapter which converts the FM reception into DAB but the reception is poor due to the aerial not being DAB - does the ND aerial fit ok ? Thanks !
Yes it fits ok, but likely needs further modifications to the antenna amplifier as laid out in the miata.net forum 
I got someone with better soldering skills than me to modify the antenna base, but alternatively you could get the base and antenna and swap both. Used from eBay aren’t too expensive.