Lighting Baxi WM532 RS CHP?

Hi there, I am wondering if you can assist me, I have been trying in vain to relight the above model boiler. The boiler ignites when the grey button is pushed in and black button pressed, I have tried holding for 20 seconds (and on occassions a lot more than this) but as soon as the grey button is allowed to depress, the pilot light goes out? any help would be greatly appreciated, kids are freezing?