Thanks for getting back to me so quickly Mike. I will definitely read up about the different methods of adding sources and I will get back to you.
Just to explain how I got to this stage with so few birth sources, when I started my tree many years ago I learnt how to input enough data to create the family records and I always intended learning how to add sources but somehow time went on and I hadn't done that. Eventually I thjought I would add media and link photos etc. and I didn't have any problems. When I finally looked into sources, a long time ago now, I did consider using AS but at that time I think I was confused by what to put in which field and I decided I preferred to just carry on inputting everything person by person.
For the next few years I added a source when I had one (e.g. a certificate) and sometimes I just added a note to say where I had found a particular record or what the census reference was. Over the years I bought certificates and so much more information was made available over the Internet that could be downloaded so I started a tree on Ancestry with the idea I would only add people to that as I verified them and amassed the census entries etc. I am well aware I should have been adding sources as I went but I often felt truly overwhelmed.
I am pleased to find I hadn't gone completely wrong by having blanket sources for each census etc. I guess I must have read that from the instruction manual at the time. I will reserve final judgement until I have followed the link you sent me. It would be great if I didn't have to delete too much and could rescue my media/source lniks.
Thank you for your offer to help once I know more about sources.