Flew at 4000 feet to get above the bad vis at the time of the flight, however still had a reasonable visual of the ground. Flew using GPS but kept an eye on the ground to make sure I was heading in the right direction. Dropped down at the end of the flight expecting to see the air field and could not find it. Roads etc all tied up with air map, or so I thought. Eventually found the airfield by back tracking, using visual referance points and roads. Apon landing I checked the airfield co-ordinates I had used and all seemed correct. However put in go to Sandy only to find that Sandy was 26 miles away. I was at Sandy!!!! Have since re checked input co ordinates and x refed with google earth and all seems okay. Have taken a bearing from my home using the GPS and x ref with air map and it still puts me out. Have checked all my other saved destinations the same way and they are spot on. Put the flight trace on a GPS plotting programme and it had Sandy plotted 26 miles to the east of where it should of been. Lucky it was not 26 miles south as I would of been into Stansted Airspace.