The message
from "The Medway Handyman" contains
these words:

Any way round this apart from a separate waste?

Take it in down stream of the trap. The pump in the dishwasher will
never empty completely, so the remaning water in the machine's wastepipe
will act as a seal.

Or, if you're that bothered, make an upstand trap such as washing
machines have.

--
Skipweasel
Pay no attention to that man behind the curtain.