Yes remove any seed heads that form on your onions or the bulb will put its effort into growing the seed and not the bulb (onion). You can bend the tops of the onion over at the neck once they are grown to size, don't do it now or they will stop growing.

Blooming at this point may mean that some of your sets were too large to start with. I have noticed that the Dutch suppliers seem to have been selling larger sets each year for the last several years. Next year, you may want to look around for smaller sets. Most of the varieties sold as sets ("long day" kinds) will start to bulb up after the summer solstice, but a few ("intermediate day" kinds)might be starting now. Wait until the bulbs are nearly full size, before bending the tops over--that action is intended to speed the maturation of the bulb, and to toughen the tunics. There is some evidence that in most cases, bending over the tops is unecessary, or even slightly harmful. Short growing seasons are a good reason to do it, though.

I suspect the blooming is more to do with the extremely hot weather they have been having in the south of the country. Lots of plants including veg. are trying to set seed as they think it is much later in the year than i actually is. I would hope our sets started to bulk up before the solstice as they've only got a couple of months after that to finish growing to size in. Remember our growing season is very different from yours TugB :-)

Thanks to everyone that replied, very useful, thanks again.

Good luck and remember there is always next year.

Onions are naturally biennials, MG. They grow and form a bulb one year, and bloom off that bulb the following spring or summer, Onion sets are started from seed late in the season, so they don't grow enough foliage to develop more than a marble-sized bulb when the day length is right. The ones that are peewee marble to standard marble size usually don't bloom late spring-early summer, no matter what the weather does. The ones that are jumbo marble size on up often bloom then, no matter what the weather does. So, unseasonably hot weather just accelerates what they would be doing anyway.
Without bloom, a long day onion will bulb up rapidly once it starts to. It just takes a month or two from the solstice before they are ready to harvest.

Our onion sets aren't even marble size when we buy them... think a very small galanthus bulb and I've never heard of 'long day onions'.

I've discovered that hardly anyone north of the "Mason-Dixon Line" knows about the differences of day length in onions, because all that they ever run into are the long-day onions. I had a heck of a time at work trying to find out what type my Dutch bulb supplier was sending me--one reason why I know as much about onions as I do now!

That would explain it then... we simply have 'onions' in the UK.