Just gone back to it - and found a very odd set of stats:
Creative A (original):
4,526 Clicks
3.1% CTR
£0.18 CPC
Served - 99.0%
Creative B (test):
55 Clicks
3.7% CTR
£0.18 CPC
Served - 1.0%
Like I say - this has been running for months - so it's not a blip. It seems that Google has "chosen" the poorer performing creative...
Before I contact support, I thought I'd check if anyone else has seen anything similar in their stats - or if I've overlooked something obvious.
Show Statistics for: Last Month.
Set it to "All Time" - and the other one is more popular overall - so Google's serving that one.
My mistake - but interesting to note that if a creative starts to pull a better response again - as happened here, Google doesn't turn it up again to "re-test it".
(Not surprising really - but if things have changed - it might be worth deleting and re-testing some old creatives manually).