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Working out that my multimeter isn't good enough to measure 10pF and resigning myself to swapping out a dozen various 0603 capacitors to work out which ones have been specced wrong.
I guess that's a simpler task than the swapping out a couple of hundred 0603 capacitors that I'm expecting will be the next step once I track down the bug.
Though there's a chance that'll be the fab house's problem rather than mine
The detritus from today's rework. Oh, plus a dead NFC board because it's really easy to lift a 1.34mm long trace when desoldering the 0603 cap at one end of it 
However, I found that two of the cap values were wrong. Four capacitors in total. I'm guessing the reels got swapped over when loaded in the pick-and-place machine?
Anyway, broke out the #TinyTapeout QC stickers to celebrate solving the mystery of the poor antenna performance! 
@amcewen Could you detect the difference with those low-cost smart tweezers that measure capacitance?
@cgervasi I wonder if they'd go low enough for these capacitors - my multimeter only has a resolution of 100pF and these caps are all in the 10-200pF range.
I also can't measure things in-circuit as the design means I'm measuring a bunch of caps at once; so I'm resigned to removing them whatever happens, was hoping not to have to solder in some of the known-value replacements that I've got here 