First of the annoying questions....

Car is an '04 TD4 HSE, I know it needs a new expansion tank (it's in a disgusting state) which I've found on ebay for about £13 - any tips on how to fit it? Is there any particular antifreeze to use?

Last question for now, is this any good - https://www.ebay.co.uk/itm/Freeland...X-FOR-BMW-DIESEL-ENGINE-IN-STOCK/372234529246
if I'm changing the expansion tank it makes sense (to me at least) to do this at the same time. Full set of silicone hoses is going to have to wait!

Easy, but make sure the engine is stone cold, and keep the cap fitted. Pull off the thin pipe at the top.
Unscrew the single screw holding the bottle on.
Pull off the thicker pipe at the bottom and put your finger in the hole to retain as much coolant as you can. Tip the remaining coolant from the bottle, into some container.
Fit the new bottle in the reverse sequence the old one was removed.
Refill the coolant with the coolant you saved, and top up if extra is needed.

Just don't mix different types of coolant. If it has the green glycol stuff use that. If it has the pink OAT stuff use that.

I'd read about that Rich, thanks for the heads up though I'm intending to flush it through anyway as the water level in the bucket (as my wife called it) is very low and a dirty horrible colour - hard to tell what it was originally! I have some horrendously expensive stuff designed for motorcycle use but not enough for a refill on the FL, after a bit of research I think the OAT is the way to go.

OAT was the factory specification coolant, but the system will work with either OAT or Glycol.