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If it turns out that I need to move my Asian red-tail into a tank of own, I might get a black lancer to keep with my other big cats. How would one compare in terms of temperament? And are they available in Britain; if so, approximately how much do they cost?

Black lancers can coexist fairly peacefully with other bigger cats. Not with conspecifics, though, unless you have a tank large enough for them to carve out territories. Males are especially intolerant of conspecifics.

It wouldn't be conspecifics. Shovelnoses, large doradids, large plecos, syno longirostris and a giraffe-nose. I'm open to suggestions as to suitable Asian cats.

I know I'm mixing fish from three continents. My nemurus gets along well with S. American cats but not African ones. One of its modes of attack--biting the flanks--is a non-starter when it comes to Megalodoras uranoscopus.

It's not such a good idea to put the lancer with armored cats like doradids, unless you don't mind scratches on its beautiful jet-black skin from minor skirmishing that is bound to occur. With the size of their mouths, they're not going to do much biting.
They're not really that good with much larger cats (like the ones you mentioned). Cats of a similar size (i.e. mid-sized cats) are more suited as tankmates.

I was going by the Manual of Tankbusters, which says they grow to 16 or 18 inches, but checking the Cat-eLog I see it's not that simple.

The lancers that show up in the aquarium trade are almost all <i>B. macracanthus</i> (with the odd <i>B. macropterus</i> contaminating shipments). I have never seen a <i>B. macracanthus</i> larger than 10", which means that they will not grow any bigger than that (I've seen enough lancers to know).
The only lancer that grows to 16-18", <i>B. hypselopterus</i> (the fish in my sig), will never show up in the trade. Indications are that this species dies very quickly upon capture and that they need a very large (well-aerated) tank with a substrate of fine mud, which would be virtually impossible to set up and maintain.

Since you say they're solitary, I'll keep the bigger of my H. brachysoma and swap the smaller. The big one can go in if I move my Asian red tail. (I didn't want to need to buy six of a twenty-inch fish).

Thanks for all the advice.

Just purchased a Black Lancer recently. It goes about it's own business. So far it's gotten along with all the tankmates:
zamora catfish
sun catfish
Iridescent Shark
Bala Shark
red tail shark
Silver tip shark
golden shark
Arowana