If the family were quite settled in the area, electoral registers may be a help in indentifying the correct family if they remained in the same household at any time whilst over the age of 18 (21 until 1970). Voting registers are usually in street order with no name index so do take quite a while to search through and they are held locally see
http://www.dorsetforyou.com/dorsethistorycentre/collectionsTelephone directories can also be useful in identifying an address or address to check in the electoral registers. I think they go up to about 1980 on
http://www.ancestry.co.uk The Dorset History Centre also has parish registers for a number of churches in the Portland district which could be helpful with regards to church baptisms/marriages etc..
Facebook can be useful in seeking details of more recent events/relationships if people have opted to leave friends lists etc. public.
There are lots of living people listed on GenesReunited too. If you can find people of the same surname in the same area being researched by the same person it is often quite possible to seek out the birth and marriage details from the General Register Office indexes.
Friendsreunited is another one to check out, finding all the schools in the area and then searching through those for a relevant surname.
I wish you well with your search.
Casalguidi