The first step to moving backwards is obtaining your gt.grandparents' marriage certificate. This will give you their ages and their fathers' names and occupations.
I checked out your tree and found what appears to be their marriage details:
Harold P SMITH & Vera KNIGHT AMJ (April/May/June quarter)1946 Repton Derbyshire v.7b p.1434
Using this information you can order the certificate online from http://www.gro.gov.uk/gro/content/certificates/default.asp
Certificates cost £9.25 and usually take about ten working days to arrive.
I searched for a birth registration for Harold Percy SMITH b.1925 Derbyshire but couldn't find one but had better luck with Vera KNIGHT.
Vera KNIGHT JAS 1924 Burton v.6b p.614 mother's m/name BARRATT.
Obtaining her birth certificate (from same website as above) will provide you with her parents' full names.
I've found a marriage for a male KNIGHT & female BARRATT which looks promising as Vera's parents':
George KNIGHT & Florence A M BARRATT JAS 1923 Burton v.6b p.761 but before ordering that certificate you should wait to see whether the names match the birth certificate.
Once you have the additional information from the certificates you should be able to move backwards but if you get stuck or aren't sure how to progress just shout.
Hope this helps