Hard to say what it smells like... Doesn't smell of diesel that's for sure - maybe a bit sweet?

Nope no gunk on the filler cap or in the header tank. Interesting idea about the header cap, cheers mate.

Haha that's a fair point about the injectors. Will have to check that! It looks like dirty water though... Hmmm... Lol

Nah no whooshing noises and the EGR hose has recently been replaced due to a split. I didn't think any kind of fluid went through the EGR so I was surprised to see this residue running down it...

Cheers for the tips.