sime friendly 80 boiler fault?

my boiler heats the hot water for about 1 min then cuts out if i wait about another min or two i turn tap on and it heats again then cuts out.the same happens with ch but it will stay on a little longer .i have asked local plumbers and engineers but they dont know or deal with sime the boiler is a combi about6 years old. hope someone can help regards nigel