I checked Liverpool Echo newspaper for an obt and there is none recorded so she is probably still alive.
Assuming you were adopted check out https://www.gov.uk/adoption-records/the-adoption-contact-register
Your mother may have registered an interest in making contact and would be on register.
Next place an small ad in The Liverpool Echo. Just head it "Brightman" so that anyone who is or knows or knew any Brightman in Liverpool will be attracted to read on. The echo staff can probably advise which heading has the highest readership but I would be inclined to use marriage or death as these are probably good for personal contacts. You will have to change wording if you decide to go with both. One might be "seeking information of marriage of" and the other "is Maureen Brightman still alive". Choose your own words carefully so as not to cause offence.