Vauxhall Zafira brake issue

Hi
Looking for advice please

My Zafira tourer which is the vain of my life most unreliable car I've ever owned.

Coming off motorway yesterday on way home, ( 2 mile trip on motorway) 4 miles before on normal 30/40 roads.

Pressed the brake and nothing there at all, pressed again nothing there panic set in the viciously hitting brake pedal with foot and did begin to slow.

Got it off the off slip ramp on to a side road, calmed down a bit. Creeped forward brakes then worked fine bit further worked fine. I was a mile away from home steady drive left hazards on just in case. When I was last corner into my road brakes again nothing there.

Called RAC came out depressed brakes spongy then went hard at pedal. Completed a short road test braking with an emergency brake. Brakes worked fine.
Pulled it into driveway brakes went again.

RAC then recommended bar tow to garage.

It's been in today but have not had time to look at it. Hoping tomorrow, if not gonna be next week Tuesday at earliest as they apparently are rammed Monday.

Any ideas what issue might be ?