Ask HN: Recommendations for website monitoring tools

Having been burnt this week by portions of our website going dark, and us not noticing for several hours, I'm after a service to do periodic health checks of our website; check various pages are up & serving appropriate content; then email/SMS me if not.

A quick google offers no end of people offering such services, but I can't find any useful comparisons of features or reliability, so I thought I'd try the combined wisdom of HN.

From my brief review http://siteimprove.co.uk look fairly reputable, but a touch more expensive than I'd expected.

Personally, UK-based sites are preferred, but I'm interested in all recommendations.