Next bit while I have a couple of minutes spare...
Expansion/radiator overflow pipe.
This had been 'repaired' previously using silicon sealant - this apparently worked for a while, but it's not ideal and anyway the pipe had come adrift from the expansion tank by about an inch or so.
I had a browse in the local car parts shop and the only pipe they had in the correct bore was fuel hose (it's labelled 5/16" and fits perfectly - just a bit of stretch). It's fine for this use, massively overspecced though being twinwall braid reinforced... Still, it's not going to pop!
Anyway, to get to the radiator cap I'm sure you're aware you need to move the fuel tank - rather than balance it precariously while leaving the fuel lines attached I decided to remove it completely. While taking off the fuel lines I noticed the reason behind the smell of fuel after running the engine:

Those splits and cracks go pretty much all the way through - the only reason the fuel was staying in at all was because the splits were being held closed by the bend in the hose when fitted. Both the supply and return lines were in that state right under the tank. A bit more of the same hose required then. This hose exceeds the BMW specs and is (I assume) much cheaper then buying the OEM lines - the only disadvantage is that it's not shaped/bent to fit and the BMW hose clips are too small as it's slightly larger externally (same internally, just thicker walls).
Seeing as I was working without a manual one bit was confusing - tugging on one end of the return hose didn't move the other end... A few more spanners to investigate.

Oh well...
The large pipe between the airbox and plenum didn't want to shift easily, so with a bit of wiggling I got the lower part of the airbox out so I could get to (what I discovered was) the fuel pressure regulator to swap out the hoses.

It's a bit awkward and a tight fit for hands and tools doing it this way, but to my mind it's easier than stripping out the plenum side - especially considering the hose clamps on the inlets are the crimped originals.
So, got new fuel lines run in - changed the routing slightly so it didn't kink on any of the bends. The bit going from the fuel rail to the pressure reg. 'looked' fine, but as I was there and had enough hose I replaced it anyway.
Then I got on with the original job of replacing the expansion tank hose! Again it needed a bit of creative routing - this time to account for the lack of clearance between the tank and frame rails.
I didn't take any pics of the new hoses - it really wasn't very interesting to look at. But, if anyone wants to see how I routed the water line to avoid being pinched by the tank it's only a couple of minutes to lift the tank and snap a pic.
I was going to replace the coolant, but as it only took about 1/2pint to top off the radiator (plus same again for the expansion tank) I thought I'd leave it for a little while and check for any leaks - I bought the new coolant so it'll go in soon. A 5 litre bottle of concentrated stuff worked out cheapest, so that'll do a couple of fills.
Costs so far:
Fuel to collect = £90
Purchase = £250
Seat cover material = £3.74
Fuel hose (also used for water) = £6.71
Hose clips = £2.69
Antifreeze/coolant concentrate = £18.56
Total = £371.70