I have a Barrus Shire which has shown something of the same characteristic. I did all the usual checks, and found that no matter how much hand pumping I did, I couldn't get fuel through. So I sought advise, and learned that the fuel outlet from the tank (in my case a Liverpool Boats) is a dip tube arrangement - ie has an upside-down U-bend - to prevent the tank draining in the event of the fuel pipe rupturing. This would explain why the fuel outlet is about 15cm from the bottom of the tank (equivalent to 50% of its capacity) while the fuel pick-up point inside the tank is probably only 5cm from the bottom. In hindsight this arrangement must be prone to an airlock in the U-bend, especially if the fuel level is below the outlet (ie between 5 and 15cm from the bottom). The fix for me was effected by an experienced marine engineer; he filled the tank and then went through a lengthy process of bleeding air from the fuel system - mostly with the engine running to ensure there was good suction in the fuel line. So if you have a similar arrangement, then you might have a residual vapour lock in the dip tube - as the tank drains the pressue of the vapour would fall, it would expand and eventually turn from a partial vapour lock to a complete vapour lock!
Mike_N