Rode the Golden Flandrian event the other weekend and couldn't believe I was the only UK rider in evidence. It's one of the most stunning events I've ever done, but pretty low-key, only 124 starters. It's not exactly a sportive - no marshals just a permanently signed route.
It combines the 3 loops that start from the Museum of the Tour of Flanders (CRVV) in Oudenaarde, that exist as permanently warmarked routes designed to take in all the climbs and cobbles that the tour of Flanders makes its selection from. Nor is it an audax as such, though there are checkpoints where you have to get your entry number stamped. It is just a fantastic day out.
I'd hugely recommend it for anyone looking for a challenge. 250km, 25km of cobbles and 25 climbs (somewhat over 2500m), mostly short and sharp.
for more info. Maybe see some others there next year? I definitely want to go back again.