Lime is available from any builders merchants, and many diy sheds,
such as B&Q, but not wickes. Its hydrated bagged lime.
There are 2 ways to use it
1. Mix lime to a sloppy putty with water, cover it airtightly, leave
it at least a day, then mix with sand to use.
2. Mix dry lime powder and sand 3:1, add water and use right away.
When the mortar is green, ie solid but not hard, brush the surface to
remove excess whiteness. This exposes the aggregate.
Lime has a few advantages over cement, and the disadvantage of slow
setting. It should not be frosted while setting, so exterior lime work
is usually left till a bit later in the year, or covered with a sheet
for a few days if frost is expected.
Dont mix cement in with lime, as most cement:lime mix ratios are prone
to cause premature failure.
Forget the posted tubs of expensive lime mortar, there's always
someone willing to pay more for something. Lime is cheap.