Feels like I've spent half my day on the phone discussing hydraulic fluid.

. In an effort to get a balance between suitability, availability and sensible price, I've spoken to another MB dealer, Febi tech support and several local Indy suppliers. Here's what I found:
Febi
Started off recommending only 21647. I knew that none of my local suppliers had this so I pressed them further. They then conceded that 21648 would be fine (though apparently not as good as 21647). I knew that I could get 02615 cheap, so pressed them for an answer on this; they said I should definitely not use it.
MB dealer 2
Initially sounded more helpful and knowledgeable than MB dealer 1, but subsequent discussions disproved this theory

(eg they didn't believe me that I had only one reservoir to serve both power steering and SLS)
Initially told me to get 0009898803. A later conversation with another member of staff identified that this is only for power steering and NOT for SLS.
I had discovered that Febi 02615 is a match for 000989910, so I asked them to check this for my car. They told me not to use it, so at least I could corroborate what Febi had told me.
Later, I rang SWMBO at work to ask her to check the reservoir cap at lunch time. After the obligatory step-by-step about how to open the bonnet, the label said 0019892003. This cross-references to Febi 21648. This corroborates what Febi told me.

D Now to find some.....
ECP had none, but managed to obtain only 1 litre for about £8. And I managed to find another place who also had 1litre for about £12 (couldn't get them to match ECP

). So now I have 2l and hope it's enough.
Phew.......