Thanks AO. I've tried the rubber covers, but the switch still sometimes switches without me knowing, and the rubber cover sticks to my credit card wallet and pulls everything out of my pocket!
I might try using a little blob of silicone sealant. A really tiny strip of heavy duty double sided tape worked for a while, but eventually the tape came out.
Why Apple can't give an option to disable the external silence switch in the iOS is beyond me. It seems that plenty of people have the same problem. Missing calls and messages is a pretty major issue for a mobile phone, I'd say.