In article ,
guv wrote:

OK thanks. I did say I thought it was - though the roadworks part
does bring in some doubt as to where the blockage actually is!

Since it is a partial blockage you can do a rough
guesstimate of where the blockage is.

Leave the drain to drain away overnight.
Throw buckets of water down the drain, until it
back-fills. Count the buckets used. Wildly guess
about 2 buckets per metre, depending on the dia
of the pipeworks.

Since the output from your w/machine is enough
to backfill then the blockage will not be far
down the pipe. Maybe a simple rodding will
send the problem away. Perhaps consider buying
a set of rods and doing it yourself.

--
Tony Williams.