<< I was a good few miles away from where it showed me.>>
There are only two possibilities that come to mind - either your GPS is faulty or it was being screened somehow to prevent it receiving enough satellites. Modern GPS receivers - even the little cheapo ones - are exceptionally accurate. If they have EGNOS capability the accuracy is down to a few feet.
For Garmin users, the company has an extraordinary after-sales service. I have twice had faulty equipment exchanged for brand new, the last being a GPS12XL which was 12 years old. They delivered a replacement by courier within 3 days with no charge. Might be worth JL having a chat with Garmin...
IO540 wrote: "He may well tell you that you are infringing now. But you could be miles out, just messing about with your track, making him nervous. I happen to know this first hand, from a certain large UK unit. "
Well, I was a Heathrow radar controller for 31 years and spent thousands of hours talking to clockwork mice on 119.9 and I find that statement very, very difficult to believe. It's ludicrous to think an air traffic controller would make such a statement. If he was in contact with the pilot there would be no reason for him to get "nervous" - ATC people aren't like that! ATC radars are very accurate and if a pilot was "miles out" a controller would NOT tell him he was infringing..