I've just tried that contact form myself.
Putting a tick in the "I'm Not a Robot" box should generate a set of photographs, where you're required to select the correct ones. (I got 8 images and 'Select all images with shop fronts'. Trying again produced 6 images with 'Select all images with street numbers'. If you're unsure about a particular task you can click the 'recycle' symbol at the bottom left to get a fresh one. Once you've made your selection, click 'Verify').
If putting a tick in the box doesn't generate the set of pictures, it's probably a browser problem. (Either you've not got JavaScript enabled or your that website simply doesn't like your browser). Try a different browser.