Hello, I hope the following assists you.
If you're certain this only happens as the fuel gets lower then the first thing to check is the fuel pump pick up in the tank. The fuel pump sits in a swirl pot in the tank, it clips in place to stop the pump moving about in the tank and to stop too much fuel slosh around the fuel pick up, if the swirl pot or pump have dislodged then it is possible as the fuel level drops and the fuel sloshes in the tank air will start to be drawn in with the petrol. At lower engine speeds this may not cause a lack of fuel but at higher engine speeds and loads it may be enough to cause a hesitation.
If the lack of power isn't related to the fuel level then on this engine I would make sure the ignition system is in good order as the ignition coil can give problems, again more so under a high engine load as a higher voltage is required from the coil.
Start with these points, let me know if you have further questions regarding this and I'll reply as soon as I'm able to.
Thank you for the fast reply, it's definitely when the fuel is low, it runs lovely with a full tank, I'll get some quotes for a replacement fuel pump. Many thanks for you help.
no problem but whoever is fitting the pump tell them to make sure the swirl pot is secured to the bottom of the fuel tank and the pump clips in correctly.