Lesley, I am sure this is an important thing for you all, as it must have been for your father. From what you have said, sounds like you are in England, which makes it more expensive to do the type of research you are planning as you have to order most certificates given the likely period you are looking at.
Be careful where you are ordering certs from and that you don't incur additional charges. The best place to order direct from on line is from
www.scotlandspeople.gov.uk where the cost of ordering a certificate (outside the period that you can view images on lines) is a flat £10 for any one birth/marriage or death cert.
Current cut off to view images on line are:
Births - 1909, due to 100yr privacy rule. Increases by one year each year.
Marriages - currently I think 1934
Deaths - currently 1959, increasing by one year every year.
After these years, certificates have to be ordered.
Something to consider for you is a trip to one of the genealogy centres (the main ones being in Edinburgh or Glasgow) where you are able to view images on line for certificates after the standard cut off dates that you have, as shown above. For a day's booking (varies but c. £14) you can view on line as many certificates as you possibly can on the day. Might work out cheaper in the long run, so something to consider in your research.
Wish you every luck

Monica