Thank you such a prompt reply. I'll get on to ordering the will and death certificate too and see what they throw up.
I already have his daughters birth certificate from 1855. But I can't find the address in the 51 or 61 census so they may be missing. I can find the address named in the probate record which was occupied by his executors, his half sister's husband and his half brother.
So the picture of Alexander's life is building up. Maybe I can find enough information to track down his marriage to Isabella Marian Keane/Keene.