2L Tdi Starting From Cold Issue

[TheBanksman]

Hi,

My S Max 2 litre TDI has recently needed the engine turning over three or four times before it catches and starts. It was always just once before (including in the winter last year). Appreciating as cars get older it may be nothing, I just wanted to try and pre-empt any issues by seeing if I could see what's wrong.

I'm a mechanical novice at best but from peering in to the engine I could see a couple of clear tubes (in the picture) one top right and the other middle left (connected to each other of course) and I can see liquid running through but there is a constant flow of bubbles in it. Is it diesel and are the bubbles meant to be there? I wonder if this may be the start of an issue with a diesel pump or if there is air getting in somewhere it shouldn't.

Any comments appreciated.