Thanks for the responses. This all seemed like a muddle to me so I asked BMW UK to clarify things. They eventually confirmed that picture A is the slide (or slider) rail and this is the part they recommend be replaced every 60,000 km/36,000 miles, together with the chain tensioning rail lining (AKA tensioning 'strap' or tensioning 'bar' or 'tensioner' lining) and the thrust piece (AKA thrust 'block' or chain tensioner thrust 'adapter') - three parts in total. So their Electronic Parts Catalogue (ETK) is correct and their Repair Manual (at least the version I have: UX-VS-2, dated 7/99) is wrong.
They also sent me a copy of more detailed service instructions (see the attached document UX-VA-4 Tensioner.pdf) that unfortunately contains the same naming errors as the original Repair Manual. The first page correctly details the "core activity" as:
(-) Replacing liner of chain tensioner rail, chain slider rail and thrust piece
but then on Page 3 the guide rail is called the slider rail (at the top) and the slider rail is called the guide rail (at the bottom). What isn't I think in doubt is that picture A shows the part that they recommend replacing, not picture B.
They told me that they "are aware of the issue and are looking to resolve the matter in future publications
" and that the error "is due to different publishers using different terms in their publications
". Maybe the error occurred during translation but I don't have access to a German version of the Repair Manual to check.
[Digression: This is I think what caused the error in the cylinder head gasket sealant recommendation. The German manual recommends "Drei Bond 1209" (their Silikone product range is available to download by clicking the Produkte link) and it was translated as "Three Bond 1209". Unfortunately there is actually a company called Three Bond and they also make a 1209 silicone-based sealant, but they don't have any relation to Drei Bond. Three Bond told me that they receive lots of calls from people looking for the BMW-recommended sealant.]
An additional piece of information that BMW UK sent me was the following screen shot of job code 11 31 705, which tallies with the "Core Activity" of the service instructions (job code 11 31 511).
To summarise, these are the three parts they recommend replacing every 60,000 km/36,000 miles:
• Chain tensioning (rail) lining (AKA tensioning 'strap' or tensioning 'bar' or 'tensioner' lining) Part Number: 11 31 1 465 189
• (Chain) Slide(r) rail Part Number: 11 31 1 465 011
• Thrust piece (AKA thrust 'block' or chain tensioner thrust 'adapter') Part Number: 11 31 7 666 337