potterton profile 40 NG intermittent firing ?

my potterton profile 40 NG boiler has recently developed a fault where the fan intermittently fires up as though the boiler is going to kick in, but fails to fire up completely, the pilot light doesnt come on either during this. This happens after about 20 mins of normal functioning. If i then switch the boiler off and on at the control board then it fires up as normal, but after 20 mins the problem reoccurs. any help/advice much appreciated (physiotherapy advice welcome in return) cheers, Tim.