1 package of ginger nut biscuits for the base, crushed and mixed with 3oz of butter
2 tubs of mascapone cheese, the zest and juice of 2 limes and 3oz of icing sugar.
I do this, (I think it was originally a Sainsbury's recipe card when I was at uni,) but I usually substitute one of the tubs of mascarpone for one of ricotta (so half mascarpone and half ricotta - the 250g tubs.)
I also usually use 3 limes, as it makes it a bit sharper, which I like, but basically, you do it to taste, as it doesn't bake. It makes it a bit more liquid, but you just mix the cheese/lime juice/sugar for a bit longer and stick it in the fridge. If you do a taste test on the mix, remember you do need to leave some to make up the cheesecake...
I tend to just decorate it with curls of lime zest.
Having a sponge cake bottom is not a good improvement. It needs crispy biscuity-ness.