I've just put some short sunflower plants in but my expectations are low but it all depends, I think, on what happens, weather wise, with the rest of the year. I've just put some broad beans and french bean seeds in which should give a late crop so I'd say give it a go. In gardening plants have two choices, grow or not to grow.

I know the climate is a shade warmer here and the season longer, but I'd say you have masses of time still for sunflowers. Late sowings tend to catch up with earlier ones, both because the seeds germinate more quickly in the warmer soil, but also because the days are starting to shorten after June 21st. Sowing now will give a good display in late summer and autumn.
Likewise things like French beans.. I sowed some climbing ones as late as July last year and although the crop didn't last long, as the frost intervened, I had a load of fresh new beans just when the others were giving up.