What does "Do a Devon Loch" mean?

Solution 1:

It's a reference to a racehorse, best remembered for the 1956 Grand National. Who as that Wikipedia link says...

was on the final stretch, in front of the royal box just 40 yards from the winning post and with a five-length lead, when he suddenly, and inexplicably, jumped into the air and landed on his stomach, allowing E.S.B. to overtake and win.

By extension, as per Bank Soal & Strategi, TOEFL...

do a Devon Loch (UK)
If someone does a Devon Loch, they fail when they were very close to winning

I freely admit never heard of it before searching Google Books just now. My figurative expression is fail at the last hurdle.