Andy ,
The pir you have in your garage is a Dual Tech pir, (passive & microwave). It is strongly recommended that you get one of the same type as a standard pir may cause false alarms due to the area of which it is situated.
The pir which you have got is made by Rockonet and can normally be purchased from an electrical wholesalers (£20 ish)
Regards
Johnny
February 2008