Keith, I imagine you're not alone in this, however it's hardly surprising that many people may be out on a Saturday, assuming they work during the week, this will be a time when they have limited ability to do their shopping or go out with their family etc.
What other times/days of the week do you usually canvass? I know it's hard work, but if you want to reach all of the people, or at least as many as possible, then you need to be very flexibile on what time you try to call. I would have thought early evenings on all days would be a good time 
Dave Wood has summed it up pretty well. The difficulty with calling before 6pm midweek is that people tend to be at work or traveling back home, the problem with calling after 6pm is that until the end of March it is dark/dusk and people don't like answering their door. Most Council meetings are also held from 6pm until 8-9pm or later which can take out another 2 or 3 evenings some weeks.
People are generally out on Saturday mornings so late afternoon tends to be the best time in West Swindon, avoiding major football / rugby matches etc... Sundays are also generally acceptable in West Swindon.
Coupled with running a chess club at the local school and having two young children a balance has to be struck somewhere.
If you do live in the Shaw and Nine Elms area and are keen to sound me out in advance of the May elections then please get in touch to arrange a convenient time.
Moley - I'm happy for people to raise their concerns and objections, I find it helps me maintain balance and a broader perspective rather than blindly following prescribed political dogma.