I've done a search and found a few but I'd appreciate comments from anyone who's using a monitoring service or software.

I want a continuous monitor on one of my sites.

The facility to add another 3 to 6 sites at various times for a week or two, and to swap them for other sites, would be handy but not critical.

This looks okay for what I want http://www.siteuptime.com/compare.php

Anybody know of anything better or cheaper?

Thanks

or cheaper?

host-tracker.com ; you can't beat free. It's like pizza, even if it's bad it's still pretty good.