Policy Statement on Address Space Allocations

Thankyou for the first constructive workable suggestion had so far. However,
this has two problems.

You're welcome.

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 :frowning: 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).