Further update. As the death certificate didn't provide any details for my birth mother, I contacted Bury Archives through the online system. They have come back to me with the index references for 3 possible matches for marriages. I'm now working on narrowing these down as the Archives offered to do a parish records search for me which is a great help!
Had a further reply from Bury Archives which wasn't as promising as hoped. Turns out they need to know the church where the marriage took place in order to be able to help - The marriage records for the 1970s are still held by the churches....MK
Thanks Margaret, didn't think of approaching the register office. Will give it a go..Unfortunately, so far it seems that all the possible marriages that I have tried have not been transcribed to Lancashirebmd....