it looks so far as though the poll is heavily in favor of "fil-lay" over "fil-let".

No, no, no! The poll is heavily in favour of "vall-ay" (or even "val-lay") over "vall-et".

"Fill-ay v fill-it" is a totally separate red herring / kettle of fish (so to speak!)

I am British.

I say "vall-ay"
I say "fill-it"
I say "fill-ay mignon", but in this case I (think I) spell it 'filet'. So as far as I'm concerned it's a separate word, although cognate.

(How am I doing at confusing this thread? )