Retroffiting buried pipes that carry hot water?
A fairly common approach these days is to make a box from rigid polystyrene (12-25mm thick) and fill it with closed-cell spray foam (polyurethane.) Don't forget to separate the pipes with more insulation - in many of these systems the double-pipe run is too closely connected and acts as a heat exchanger - not only does the hot supply drop in temperature, the cold return rises in temperature from house to heat-source. 

That heat is not technically lost, but it's also not useful/available for heating the house. But you might want to measure it and see how much you are really losing, if that changes your mind about digging up the whole pipe run.

This foam box and spray foam approach does not require cutting the pipes, nor doing a completely new installation - just digging for access.

The box, in cross-section, would look like an "E" laid down flat - you put one pipe in each channel and fill the rest of the channel with spray foam - most people put a foam lid on the box ("E|") after applying the spray foam, and then backfill.