worcester 15RI lockout problems?

hi,I need help with my greenstar 15 RI which keeps going into lockout. The boiler is just outside guarantee,and has run faultlessly for two years,before breaking down over xmas.I've reset it ever since and got by,but it is getting me dowwn.I reset the thing and it will run find sometimes for 1-2 days, other times for anything up to three weeks,then no hot water or heat,and the boiler has gone into lockout,shown by the blue light being off flashing on tiwce a second..I've checked everything in the manual that I could check,but really need help to fix the thing.I suppose I could go on resetting it every time but it's a bind as the boiler is in the loft!! help