Moon herb ...dont think you need do anything with a Garmin ( see attached extract last line ) and link to complete article
http://gpsinformation.net/waasgps.htm
"This does NOT mean that you can be in England, Mexico, Canada or any other location very distant from the WAAS correction stations and expect the USA WAAS corrections to be useful even if you can receive the WAAS satellites. In fact, if you are in Europe, Africa, Australia or anywhere more than perhaps 500 miles away from the shores of the USA, the use of WAAS signals may well REDUCE the accuracy of your GPS fixes. Europe's EGNOS system (compatible with WAAS but for operation in Europe) is in the test phase now and is transmitting a "do not use" flag. Magellan GPS receivers ignore this flag and use the signal anyway (for what it is worth), but Garmin receivers honor the flag's intent and will not use the EGNOS data as long as the "do not use" flag is set."
regards FFB