Old Vaillant Combi boiler pilot light?

I have an old but until recently perfectly running Vaillant combi boiler, but now everytime we want hot water or want to put the central heating on (as it is August!) the little black (ignition?) button keeps throwing itself back out. The little blue box underneath where the spark is, keeps flickering but never seems to 'catch'. You have to keep re-setting (pushing in) the black button to get a pint of hot water. It sounds just like it is firing up, but then that black button just pops out . Any ideas which part is not working properly? All ideas gratefully received.