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