I was in a similar situation about 15 years ago and had to buy six certificates (at £6 each then). I have also been to Dublin at the old Joyce House and not only benefitted from the £1.50 for a plain paper copy (which is all you need) but also from the 15 minute turn around they had then. UK Government take note - but they never will.
You need to be sure that you will recognise the correct Mary if you do order all seven. Is it through the father? You may be able to specify the bride's father's name and get your money back if this does not match. Personally I would apply in writing with all applications in the one envelope, so the GRO in Southport know you are only after one certificate.
If the not to the GRO, but to the local register who have the original certificate. Of course you have to apply by post to seven different Registers, and stamps are not cheap anymore.
If you have an address for her after the marriage you could try the electoral roll or a POst Office directory; or if you have an address in 1938/39 look at paying £45 to the NHS for the 'census' done as part of the National ID cards. You will get the names of up to ten people in the house.
If you know when she died you may be able to find her death on FreeBMD using the age you have plus the seven alternative surnames. At the very least you may come up with two favourites and apply for these first.
Ken
