Hello Gareth,
I was able to find the problem; the fix is already merged in master and 0.19.x release branch.
Also, I just released Hawkular Metrics 0.19.1.Final with the fix. The build should be picked up in the next Hawkular Services release. If you do not want to wait that long you could get the binaries for Hawkular Metrics 0.19.1.Final and manually update the Hawkular Services distribution.
Here is where you can find the binary distros for Metrics: http://origin-repository.jboss.org/nexus/content/repositories/public/org/hawkular/metrics/