This topic has the hallmark of bambergaspipe. For those who don't know it was a university psychology group who were testing peoples reaction to plausible faults, and the replies went on and on. With increasing frustrations the dipstick group presumably thought they were discovering something worthwhile.
Three months to clear a blocked pipe is ridiculous, come to that, so is three hours.
As for the current blockage, it is quite likely to be ice in the fuel pipe. Either water got in via the the sight tube, where the top is exposed. or water was stirred up in the tank when fuel was delivered.
Remove the filter, connect a hose to the boiler end of the pipe, hold it up high, and pour hot water down it. When the ice melts it all flows out of the filter end. Then flush the pipe as normal, but do stop sucking the end of the pipe, buy an oil syringe.