Thank you for these answers. I'm currently waiting for Dad's mother's marriage certificate (she married when he was three years old) and hoping that it contains some details of her family members so that I can attempt to find out a little more. I may, at some point, need to pay a visit to the local record office and see of I can see an registers of either the workhouse or the London Orphan School, but I need a clearer idea of where he was placed during those three years. Roll on 2021 when the census may be able to help me a little more.
Think I'm going to have to read up on workhouses in general. Seems all we're told, for the most part, the very worst parts of some of the earlier workhouses - the 'Oliver Twist' stories. Jennifer Worth, in her 'Call the Midwife' books discusses some of the more humanitarian practices of the later workhouses and tells the stories of some people who actually got their start in life and a decent trade thanks to their workhouse childhood.
I have read that by the time Dad was born, Watford Workhouse had stopped putting the title 'workhouse' on birth certificates so as not to hinder its inmates in later life. Instead they just put '60 Vicarage Road', which seems like a very forward-thinking thing to have done.
Not an ideal way to start or end your life, I suppose, but marginally better than starving or freezing to death on the streets.