Repairing a dry stone wall, the easy way?

[Crofter]

A few years back I had a pile of very low grade rock left over from some landscaping work, and used it to build a dry stone wall in the garden. 

It's crumbling quite badly and I think one section might fail soon.

I've tried rebuilding it in the past, using mortar to shore it up, which worked fine but was very tedious. I think the current section might be beyond that! 

 

It's only a low wall, built from two 'skins' of stacked stones with a rubble infill. I'm thinking I could just 'inject' some sort of sealant/adhesive in lieu of mortar, in to the gaps between the stones. Not sure what kind of gloop would be best though. Has to be a neutral colour and weather resistant. And fairly cheap...