I believe that Blakes and Hoseasons in the UK are agents for canal boat hire companies using the Canal du Midi. I appreciate that you may pay extra for the privilege but you could try looking up canal hire boat companies on Yahoo.fr - I think there is a company called Blue Crown Line that exist ....

Just a couple of pointers (I've done plenty of canal cruises in the UK), here in France the lock keeper works the locks for you ('fraid no DIY) and s/he has set hours, including time off for lunch (often up to 2 hours!). You'll need to try to work your schedule to include lock opening hours (lock=ecluse, lock keeper=eclusier) to plan your route/stops/daily distances. Also normally canal boat hirers need a canal driving licence (yes; like one for a car with a formal test before it is issued), but by going through Blakes/Hoseasons and co, you won't need it as they have a special 'loophole' in their contracts with the French canal boat hirers trade union etc so you are exempt or hold an 'honorary' licence for the period of your holiday.

Canal boat hire sadly is not cheap, but it IS fun .... and get in plenty of mozzie sprays and creams because they have gastronomic feasts on uncovered flesh, especially when British!

there are lots of hirers on the Midi Crown blue line, connoisseur boats are a couple of UK ones Locaboat and Nichols are some of the large French companies but there are lots of others. the weather is usually warm up until the end of October. the locks are a combination of self operated and lock keeper assisted. I would say the main consideration is where to start your hire. some bases are very rural like homps or in the town/city like Narbonne or Carcassone. the canal gives you a great range of places to visit like the walled medival town of Aigues Mort or the beaches of Palavs
where ever you chose i'm sure you will have a great holiday. if you need more specific information abou the area please contact me directly