Low fuel level

Hello I’m looking for some advice on my Ford transit custom 19plate. I went on holiday for 12 days came back turned the van on and it said no fuel, I never let it go low so drove it to the petrol station put £20 and the needle didn’t move. Took it to the garage they said it was fine to drive but drop it in when you have time. When I was driving it started to pulse and went into limp mode. They replaced the fuel sender unit and it worked for 1hr then fuel level low again. Turned the van off gave it 5 mins then back on and it drove fine for roughly 4 days. At the end of that week it dropped to fuel level low again but after driving for 10mins it went back up and was fine until I turned it back on the following morning and it was right back down at 0 again ! They have been sent a wiring diagram from Ford and can’t find any wiring issues so they have now sent the Speedo head for a diagnostic check ? I have now been told the diagnostic check has passed and there wasn’t any issues with the Speedo head. They have now said they can’t go any further with the van and it needs a full system update ?

Thank you