RX 40 are rubbish, we use Rokonet 900PQ for all our detectors and use the DT models for commercial etc.
We use the Optex FX50SQ for conservatories and they are a great detector.
A few of the //.National Installer.// guys have mentioned the Aritech ultrasonics in the past too.
Lets be honest they must fit bucket loads so they must be well proven.
Paul