a note of caution; the heat transfer rate from the fluid through the wall of the pipe will vary with the flow conditions and the material surrounding/in contact with the pipe.
It is possible that the flow is changed by travelling around a 90 degree bend such that the heat transfer into the surrounds is increased. If there is damp material in contact with the pipe at that point, the rate of heat transfer out of the pipe will also be increased.
However in the absence of a better idea, or a leak that is more obvious somewhere else, you are maybe going to have to bite the bullet and start to dig stuff up sooner or later.
BTW do you think the leak is such that you would be able to hear it/find it etc, if the system was drained down and a slight overpressure of air was used?
Also, is it worth trying a sealant in the system? Fernox make one. Even if this worked well enough to get you through the winter, it would allow the repair to be delayed until it is more convenient....?
cheers