According to the advice on DEFRA's website I've been reading this morning, you should be making every effort to make your garden unattractive to wild birds, and discourage them from using any bodies of water.
https://www.gov.uk/government/news/avian-influenza-bird-flu-in-winter-2016-to-2017
I'm currently outside the high risk zones, and I'm trying to make a decision about letting my girls out again once the measures are lifted at the end of the month. I want to let them have a bit of freedom again, but in the high risk areas they are advising some areas to maintain max biosecurity and housing until the end of April! :/ so I don't think the threat is completely over yet.
There isn't a major threat of Avian Influenza- DEFRA are just being overly cautious because they don't want an epidemic...... During Foot and Mouth, out of the millions of animals that they slaughtered and only 20% actually had the disease, which just shows that they do sometimes go a bit overkill.
If I was you, I'd just let my ducks outside- there's a TINY risk of them becoming infected, and as long as you are being careful (such as covering up food and not introducing new birds) then I'm sure that they will be just fine.