Thankyou for the first constructive workable suggestion had so far. However,
this has two problems.
a) RIPE fidn't give me the first /19 in a shorted prefix block
( its x.x.160.x and .192.x is used), but no matter, I'll renumber
if necessary or persuade them to give me a /18 as well so I
can do the above (hopefully).
My convert the /19 to an /18 was a way to get minimal extra announcements.
Getting a new /19 and keeping the first 3 /21s for your own use and giving
them the 4th, still adds only one EXTRA announcement (over the need to
announce the [new] /19 itself).
b) The /21 advert may be inbound filtered by a.n.other, which will be
fine if it has an AS-Path through me (as the less specific route
will work the same way) but won't when that path goes through the
other provider with whom they are multi-homed, as the /21 will disappear
entirely (3rd parties, i.e. a.n.other's customers will see neither),
the /19 will be the only thing that is visible, and I'll just black
hole their packets.
As a Multi-Home (as opposed to a Private) /21 it should (theoretically) be
entitled to being added to the filter lists as valid - Getting this done is
a political problem. You should not be black holing the packets since your
receipt of them is VALID (since they are Multi-Homed as opposed to having
walked with the block).