I use 1 part 3.5NHL lime to two parts sand. The sand should be well graded sharp sand with plenty of fine stuff. I used to use sharp sand from Travis and add some stone dust, which I got from the local limestone quarry. Now I use a mix of sharp sand with a bit of plastering sand in it.
I mix mine in a Belle mini mix and throw in a couple of grapefruit sized stones to smash the mortar when it's mixing and gradually add the water, leaving it a bit for the mortar to mix well and then add a splash of water now and then to get it nice and sticky. It's easy to add too much water and if you do you'll have to add more lime to soak it up.
I don't make lime mortar out of putty lime, as it has a weak crushing strength and no frost resistance.
Once mixed, the mortar should hang off an upside down trowel. It starts to go hard after a while in the barrow, but you can smash it up with a hammer and it'll go nice and plastic again. The mortar has two sets, an initial set which can be broken and a hydraulic set which takes about a month. If you're quick with laying the stone, you can take advantage of the initial set to build slightly higher in a day.
The lime I use is Brekweg, but I'm not sure you can get that in the UK any more. I got mine direct from Germany for the house I'm working on at the moment, but I have bought it from Mike Wye in the UK before.