That plan sounds sensible to me and tackles the likely causes rather than just moving water around. A few things I would check while it is open. Make sure the shingle margin is genuinely 150mm below the DPC along the whole run, and that the trench base falls slightly away from the wall so water cannot sit against the brickwork. Lining the sides with a geotextile membrane stops the sand and soil migrating into the shingle and clogging it over time. On the vent, one telescopic vent will help, but it works best if it gives a clear cross flow path to the existing airbricks, so check nothing is blocking the void between them, such as old rubble or sleeper walls without gaps. Fit the new joist ends on a DPC strip if that was not done. When digging near the conservatory, do test holes first as the footings may be shallow. If anything looks structural, get a builder or surveyor to look before going further.

The survey plan looks like the sensible route, and I agree with JpManning about checking the fall and the depth along the whole run before it is covered up. I would also watch how it behaves after the first heavy rain, as a trench that looks right in dry weather can still hold water against the wall if the base is flat in places. If you do add a channel later, make sure it has a clear outlet and can be cleaned out, or it will just become another place for water to sit.