Sounds like the sender unit windings need cleaning, It sounds daft but the sender windings become coated in a sticky fuel residue, considering it spends most of its life submersed in fuel you wouldn’t of though that possible. You gain access to the unit by lifting the carpet up in the rear to expose a round galvanised plate secured by 3 cross-headed screws. Before you start! disconnect the battery, (sparks and fuel don’t mix very well apart from inside the engine.) Remove the plate and you will see the top of the fuel tank and the sender unit, before you remove the unit disconnect the plug and match mark the unit so you can fit it back in the same way you removed it. Undo the brass securing screws and lift out the unit, you will now see the coil and wiper that moves across the coil when the float moves up and down. Very carefully clean the coil with alcohol; I used the wife’s nail varnish remover and a cotton bud. Re-fit and hopefully it should be OK. I think the gasket between the unit and tank is made from reusable silicon rubber so you properly want need to buy another one. Uk time Friday 00.02 am