Underground oil tank and Worcester Bosch Boiler intermittent cut-out Why?

We have recently installed an underground Balmoral Bunded oil tank and a Worcester Bosch Utility Condensing Oil boiler. All ran well for first 2 weeks then cut-out. Since then we have had intermittent cut out problems and syphon system has had to be vacuum filled several times. Tiger loop initially but this then removed for just non-return valve. System pressure checked and Ok no leaks. We cannot understand how air can be getting in to break syphon each time but I am now worried that location of tank relative to boiler may be a problem. Although underground, about 1/2 of tank is above boiler inlet level and tank about 20m away from boiler. Are we asking too much of the boiler pump? Any suggestions most welcome as we are running out of straws!!