The Allium Leaf miner (Phytomyza gymnostoma) was first found in the UK in 2002-3. It can damage leek, garlic, chives, onion and other Allium species. Whilst it originally appeared to be confined to gardens and allotments, particularly in the West Midlands, it has been seen in commercial crops in the West Midlands (leek and salad onion). There are some useful links below:

Information from the RHS