One method is to use Ancestry's certificate ordering service. First go to the Births, Marriages+Deaths page.
Select Marriages 1837-1915.
Input "John Gick/1840/West Derby, Lancashire.
This gives the following entry:

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England & Wales, FreeBMD Marriage Index: 1837-1915
about John Gick
Name: John Gick
Date of Registration: Oct-Nov-Dec 1840
Registration district: West Derby
Inferred County: Lancashire
Volume Number: 20
Page Number: 699

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Click on "order a marriage cert for John Gick" and follow the instructions.

I don't know if there are less expensive ways of doing it---someone else might know.

Jean C