The causes of leukaemia can be genetic, radiation, some chemicals or a certain type of virus (HTLV).
I have never heard of it being caused by compost, so I don't think there is a need to worry about the compost unless you have a compromised immune system - if you have a worry about Legionella then wear a face mask as advised.
To think that London, or anywhere else for that matter, does not belong to any one demographic