OK, this is now sorted. For those curious, my LN anchor TX got RBF-ed into an anchor consolidation TX (part of the protocol apparently), but that TX was not in my mempool (300MB) and so LND wouldn't let me spend my output in the parent TX.

So, I increased my mempool memory, waited for it to sync (which took a while) and then boom, it appeared. LND restart and the 'stuck' TX is now spendable.

LN is a beast.