Hi
There is a problem with Shipston on Stour civil district - it covers more than one county, therefore they could have actually married in any of the parishes within three counties. So you would need to search about roughly 20 parishes for the marriage.
If you have the page and volume number for the marriage then it would be far cheaper to order the marriage certificate from the general register office than to pay for three different record offices in three counties to search the records for the marriage.
You can order the marriage certificate from the gro online
http://www.gro.gov.uk/gro/content/It may cost you more for postage to australia but record office will charge you around £20 per hour for searching all the parishes in there records that are within Shipston civil registration district, to then possibly have to pay this amount three times is going to add up to a rather greater sum of money.
If you go to this link on the warwick ro page
http://www.warwickshire.gov.uk/Web/corporate/pages.nsf/Links/8DFFB66821BBDEBD80256A9A004DC89Aclick on the pdf file that shows poor law districs ( the second one down) you will see in the south of the county Shipston district - it stretches into Worcestershire and Gloucestershire (shipston was in Worcestershire until the 1920's) you will see there are several parishes within the thick black line; your couple could have marrid in any of these parishes.
Hope all this makes sense
Lemon