[JayG]

Would I be right in assuming that the more granular it is, the slower acting? Or have a bought the dregs of something that a decent garden shop would turn their noses up at?

There are 2 basic types of lime for garden use; hydrated lime which is relatively quick-acting and ground limestone which being relatively insoluble is a slow-acting source of lime (and is usually more granular).

Ground limestone is generally preferred as it leaches away much more slowly and won't produce a sudden change in the soil pH, but then again if a big increase in pH is what you are actually looking for then hydrated lime would be the choice.