Garlic banished.

I've just pulled up ½ a dozen rather sick looking garlic bulbs only to find no root and a health population of white maggots/worms. Im guessing, having never seen them in the flesh before these are from the onion fly ? The bed has been covered since November with enviromesh to keep out the allium leaf miner so I have no idea how they got in there. 

I'm afraid for Monsieur Garlic that is the final straw.

Oh dear! Are your onions and shallots looking healthy?

I'm still playing catch-up with some of these allium diseases, especially leaf miner, but given their life-cycles it appears possible to inadvertently enviromesh them in whilst they're still in the soil rather than keeping the adult flies/moths out, although you might hope that would only be a problem if you are growing alliums on the same patch of ground as in the previous year. 

Last year was the first time I'd had pest problems with any alliums, so with my shallots affected now with ALM, leeks last year, there's plenty to think about. My garlic seems OK at the moment, but I daren't look too closely. 

Can't speak with any authority or experience about your pic Lardman, but given that the roots have disappeared but with no sign of fungus it presumably isn't onion white rot, and ALM don't eat the roots as far as I know, so it is likely to be onion fly given the number of larvae you found.