They may well come away from the base/just below ground, so trim back to about 6 inches and give them another month. if no growth visible by then then yep all dead.

Thankyou Meadowland and Seaburngirl for your advice, it is what I expected but I am still going to give it a month before digging them up. Thanks again.

I'd cut them back and hope for the best - there's nothing to lose after all. there's no chance at all if you dig them up.