If you've already got a FA Offset mortgage and a lump sum it makes more sense to put it in the offset account as the FA variable interest rate is a high 5.3% and by doing this you won't be paying DIRT.
If you've got regular income it makes more sense to put it in a regular saver account if it's one of the ones offering about 7% (5.6% after DIRT). The benefit is marginal however.
You could drip the lump sum in the off set account into the regular saver to optimise things.