Magnet has a point actually, it has been tested by a derv specialist!?!
This is where it gets difficult for a joe bloggs giving advice to a Sam Smith where to spend some money. But if I was in your shoes right now this is what I would do.
Park car nose down for at least 3 consective nights, if it starts no issues it pretty much proves fuel running back to tank issues.
Next if you want to, clamp the fuel return pipe coming from the filter. Leaving car parked flat. Remove clamp in the morning, see how she fires up.
these tests basically also rule out injector seals as the main cause. Though not saying they are 100% fine of course (covering my back there 😬)
All this confirms fuel is eventually losing the vacuum that keeps it in the pipes and mechanical bits. Now, extrapolating info from other forums, on 1.6,1.9, 2.0 and 2.5 TDI engines fitted to VW, Audi and Seat vehicles, the most common fix is the primary electronic pump fitted in the tank. The non return valve built into it being the issue.
Your talking less than £50 for the pump but shop around, e.g
https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.co.uk%2Fulk%2Fitm%2F162547351547
You might have some negotiation leverage with the firm you spent money with to contribute to the labour of doing this maybe!?!