Brandt Apollo fridge freezer sounding alarm?

My Apollo fridge freezer is having some cooling problems so I followed the advice on here and defrosted it the hard way (pulling panel off inside freezer etc)

It still isn't cooling very well and when I turn the fast freeze on, the yellow light and the green light flash rapidly and a buzzer sound intermitantly buzzes in time with the flashing of the lights. Does anyone know what this means? It's not in the manual. Low gas pressure? Thanks