Well the magnets arrived today - so installed and what a difference ..... lap times are reduced by 4 to 5 seconds on a 816cm track.
I originally tried to fit a stonger magnet but could not easily remove the existing one without fear of damage so then added a second magnet the same size as the original but it fouled on the main cog.
Ordered the small magnets as per Woodcote suggestion (thank you) and they did the trick - originally fitted 2 but not so succesful. Added a total of 4 in the end and that did it. The resistance when lifting the car from the track is now very much the same as other cars I have.
Far happier with the car now ...................