In article ,
Cursitor Doom wrote:

Questions: what makes this thing so special as to cost so much?

At a guess - exact replacement parts might no longer being made,
the equipment manufacturer has a small remaining stock, there
may be no other source. Some owners of the equipment (e.g. military
and some businesses) may have an "exact replacement only" policy
for spare parts, to avoid the need to send equipment through a
formal requalification process.

So, Marconi can charge that much for a cap, because there are people
willing to pay it (rather than scrap the whole piece of equipment).

Why have the designers used such a huge capacity cap in this low current
drain application?

Might be "because they could". Or, possibly, some of the downstream
circuitry might have poor power-supply rejection, and having a truly
huge filter cap might be the only way to get ripple-related noise
and sidebands down low enough to meet the device's specs. They might
also have figured that this part might be prone to degrade over the
years (as it apparently has done?) and they installed one of larger-
than-initially-required capacity to stave off the effect of this
aging and degradation.

If I can source a generic electrolytic of the same spec or better for
30 quid, why should I not use that instead of the bespoke replacement??

The extra hold-down terminals might be needed in order for the device
to meet its reliability specifications, when installed under
conditions of high vibration and possible acceleration shock (e.g. in
military installs, on boats or airplanes). Without the additional
pins soldered to the board, vibration could result in the cap
shaking back and forth, with all of the stress placed on the two
solder joints (and the PCB traces) resulting in stress cracking.

A standard modern cap of the same capacity and voltage rating, and
equal or better temperature and lifetime specs, is likely to be a good
deal lighter than the original. If you can find one which fits the
connection terminals, and don't mind the fact that it might break
loose if you use the equipment in a bomber that's flying through
intense flak explosions for months on end, I suspect it'd work out
just as well for you.