Boiler problem

Hi We have a problem with our boiler which several visits by a competent plumber have failed to fix. I hope that someone can point us in the right direction because it is getting expensive to solve. I must apologise for the long explanation ?.. Condensing boiler : Glowworm Ultracom 30cxi Approx 7 years old The problem is that, at certain times, the boiler will fail to ignite and it gives an F1 fault code (see below). The boiler has had this problem since we moved in to the house approx. 3 months ago. F1 Fault code ?Boiler has attempted to light 5 times and failed. Possible causes according to the instruction manual: -Gas tap is closed -Problem with gas valve connector -problem with ignitor / low gas pressure -Incorrect gas valve adjustment -Electrode ignition lead problem -Electrode problem -Air inlet blocked -Earthing connection problem -Condensate blocked In recent visits the plumber replaced the pump (grinding noise and about finished apparently). On his next visit he was going to change the electrode but in the end decided to change the igniter. The problem still persists. Apart from perhaps the electrode, we think that the other possible causes are not the problem. The strange thing is that we can quite effectively predict when the problem will occur. It will never happen on a weekday morning luckily but it regularly happens on a weekend (Saturday morning and Sunday evening particularly). Until recently, resetting the boiler got it to work straightaway but in the last couple of weeks there have been a couple of occasions when we have reset it several times consecutively because it keeps failing and then had to give up. Later on it will work again. We mainly notice the problem when running a hot water tap/shower and the strange thing is that we will get perhaps 30 seconds of hot water before it cuts off which seems a bit strange. We have only noticed the heating not fire a couple of times. Because the timing of the fault does not appear random we wonder if it could be linked to the timer. We control the central heating from a wireless thermostat but this doesn?t even have a day to day function on it. We have just noticed in the instruction manual that the actual boiler has a clock and timer but we cannot access it from the control panel (supposed to hold down Program button for 5 seconds to set time). Rather than a fault we suppose that this may just be over ridden by the remot thermostat. Any ideas what the fault is and how we can fix it will be gratefully received. If you have any further questions then please ask.