I'm copying here a response I posted to John-C, re CRIS & tagging., as it's a more appropriate 'category'.
"Hi John,
Not bothered with CRIS registration or its useless tagging system.
I've posted on this to uk.rec.caravanning some years ago; do a google-search for the old postings under electronic tagging or CRIS, or doc, etc.
Why? Because the CRIS system uses rather old fashioned technology, incompatible with systems used elsewhere. The Police don't really bother with it, because their detectors (all 30!) have to get so close to the 'van to read the tags. I've been led to understand that they don't even bother to keep the readers on charge.
I use a tagging and registration system (http://www.stoptheft.co.uk) which is far more widely applicable (on the continent as well). The UK Police have well over 1000 readers, and they can read tags without going so close to the 'van. I think the insurance people, CaravanGuard use this system and maybe Barrons.
It uses a well-proven system by Trovan ( http://www.rfidnews.com/stoptheft.html), which is used in industry and the Health Service etc., to help in prevention & recovery of stolen/mislaid items.
My Hobby is fitted with 2 credit card tags and a 'pill' tag, plus numerous warning notices.
My 'van is also fitted with a large roof notice which reads:
"If NOT towed by a
... . .Silver
Skoda Octavia Estate
{Car's reg No.}
Phone POLICE"
I've just changed the car, so I went to the sign maker guy to get the ammended parts of the notice; it cost £18. Got to fit them on before I tow the 'van!!
Police have said it's a good idea. It aids recovery, but obviously is not a deterrent. Total prevention of theft is an illusory goal; if they want it they'll get it. All you can do is make it difficult to steal and add in something which will aid recovery AND conviction of the thieves.
There is a fancy recovery/anti-theft? gadget (not Tracker) ,which uses a GSM phone and GPS location technology, and 'phones you up if your 'van moves beyond about 5m from a specified static location. I've been told you can even get software, to see, on an on-screen map, where the 'van is!!! A friend is trying to find out where you can get it and, importantly, how much it costs! The advantage over Tracker is that there is no annual charge; just a one-off purchase price.
Cheers, Doc."

