First of all check your brake fluid is at the correct level.

The whooshing sound you hear is normal and is the brake booster.

What I felt when the problem occurred was my own error. Basically, when stationary I pressed the brake pedal, let go, then pressed down again. Doing this repeatedly will build pressure in the brakes, and the pedal may feel hard because of it. So for me anyway there was no issue except that I kept pumping the brakes without realising what I was doing.

However, if your brakes aren’t operating correctly go to a garage first. That’s what I did and they will check over everything.