why does my pilot light keep going out on my vokera 20-80 rs flowmatic?

when i have my heating on after a short time my pilot light goes out on my vokera 20-80 r.s flowmatic , i try to relight it but it will not stay lit, the only way i can get it to stay lit after several attempts is to have the hot water tap running which forces the burner to fire up but after a while it cuts out again could anybody help with this matter ?

martin gomersal , wakefield
December 2008