vokera maxin 28e boiler will not fire up?

i have a vokera maxin 28e and it has been on the blink for ages it used to fire up not so long ago when it felt like it and after been on a while the flame would jus go out and then the red restart button would come on and thn when u pressed it it jus wouldnt do any thing . after hours of tryin it would eventually come bk on and repeat the whole process. and now wont fire up at all.. just wunderd if it could be any thing to do with the exspansion vessel because when the boiler is warm and the pressure rises it comes out of the blow off and thn when it cuts out and gets cold the pressure drops so am constantly topping it up . could this prevent the boiler from firing up at all.