Ok the midwife has phoned back and said ..

The aspirin is to keep my blood and flowing more freely as there is the risk that I could loose some protein into my urine rather than staying in my blood for the baby. If it leaks into my urine and suffer from high blood pressure it could lead to pre eclampsia. I haven't had high blood pressure ever before but with the recent of my history of my kidneys they would prefer that I took it.

Yeah she said to make sure you drink loads, which I do already as it helps the urine flush through quicker and the less chance of an infection. Yeah I have kidney stones, there is still some in my kidneys on the branches but not big enough to cause pain. That is the last timing I want while being pregnant as it is sore!