When id o a ride over 33 miles it overwrites the waypoints/course for the first part of the ride.
How do i sort it?

in the settings for the tracks there will be a box that says wrap when full OWTTE. make sure thats un checked.

then it will stop recording won't it?

it should then overwrite every other one or similar to make best use of space available to log your whole route. loses some accuracy, but is supposed to be the best compromise.

the sampling rate has to be set somewhere after all.

ok thanks for your help.i will try it on "smart" settings for my next longer than 3.5hrs ride 🙂

No it won't stop recording if its unchecked.