Unburnt diesel is quite 'sweet', well, kinda like barbecue lighter fluid. It'd make your eyes sting if you got a face full of it!

There is no fluid running through the boost hoses as you correctly pointed out, but because of the wonderful way EGR valves work damp sooty shit gets pulled through the engine and leaves that oily residue that coats the inside of all the pipes and intercooler. You'd be surprised at how much sooty liquid can come out of an intercooler when it's taken off!

If yours is a Euro 3 (no electrical connectors on the EGR valve) then get it blocked off! Unless you have one of the rear EGR cars - if so then I'd put money on it being the EGR cooler that's leaking into the engine, but if it's an 04 I'd imagine it has a front (non cooled) EGR.

I'm just sceptical that it's the HG as it's not a common occurrence on TDCis, I'd go as far as saying it's very rare! The oil cooler is often a prime suspect (and its failure is often confused with a HG failure) but the pressure in the engine is such that it tends to force oil into the water, rather than allowing water to 'leak' in the engine.

It could also be leaking from where the coolant goes through the turbo - that's where mine leaked. Because it's right at the back of the engine, and there's loads of stuff in the way I only noticed it when the car was up on the ramps and I was taking the cat off - I'd never have noticed it otherwise.

Def replace the header tank cap, and have a really good look for leaks. Sometimes it may be such a small hole that it only leaks when you're on the move and the fluid is heated and under pressure. Take it for a good run, and check it over carefully once hot.