Yeah - from what I understand your bank is allowed to expect you to have insurance on your bond, but you are definitely allowed to source your own rather than take whatever they're offering.
This article covers it pretty well:
http://www.mybondcover.co.za/mortgage-cover.html - "Combined with the fact that you don't get to shop around for the cheapest rates when you finance your mortgage cover payments, this could end up costing you tens of thousands of additional rands you could have used to settle your mortgage faster."
Suffice to say I went direct with Momentum - not Standard Bank's mortgage cover.