To make the cheesecake filling add the cream cheese to a large bowl and whisk until light a fluffy.
Add the whole tin of condensed milk, the double cream and then finally, whisk in the lemon juice squeezed from the 4 lemons and set aside the lemon zest to fold in afterwards (I find the whisk gathers all the zest up so its best to fold this in)
Plop all the lemon mixture into the cake tin to cover the ginger biscuit base and smooth the surface to avoid lumps and bumps.
Pop back into the fridge for at least 3 hours to let the cheesecake layer set. (It's best left over night though)
To make the candied lemon decoration simply finely slice the 5th lemon, add to a little saucepan along with 200ml water, simmer for 30mins, top up water to approx 200ml again and add the caster sugar. Keep an eye on this whilst it bubbles, shake the pan once or twice but don't stir.
Once the water/sugar mixture looks syrupy, remove from the heat and life out the slices onto a piece of baking paper, these can be kept on the side overnight to harden slightly (they will not go hard, they'll have a bit of chewiness to them)
Run a knife around the edge of the cheesecake to loosen, gently light the cheesecake out of its tin and top with the airdried candied lemon slices.