Received a very polite and helpful reply from SP.
" Where an index link leads only to a microfilm cover sheet, it typically means that the original indexers had found the corresponding entry within a different record set from the Old Parish Registers (most commonly kirk session records, though they can also include duplicate registers or local midwives' registers). Although there are no linked digital images available, these entries have been retained in the online index to help users locate further records of relevance to their research and we will look up the additional volumes upon request. If our search is unsuccessful, we will, of course, refund any credits used.
In this instance, the entry relates to a nineteenth-century transcription of a kirk session register for Tundergarth, one of the so-called 'Miscellaneous Manuscript Records', a collection comprising mainly duplicate or scroll copies of registers placed in the custody of the Registrar General to supplement the Old Parish Registers (see here for a detailed list of these records:
https://www.nrscotland.gov.uk/files/research/miscellaneous-manuscript-records-catalogue.pdf). I have looked through the corresponding kirk session record (NRS catalogue ref. CH2/831/1), which records marriages for the years 1792 to 1824, and was able to find the original entry pertaining to the marriage of Joseph Jardine and Janet Ker on page 66. Please find attached an image of this folio for you to download. The relevant entry can be found at the bottom of the right-hand page.
This particular kirk session volume contains a census of the parish compiled between 31 October and 17 November 1791, which lists the names and ages of all occupants of Tundergarth. I also looked through this later section of the volume on your behalf, and was able to find an entry for Joseph Jardine on p. 146 and potentially one for Janet Ker[r] on p. 148 (Joseph's place of residence is the same as in the marriage entry, while Janet's is merely given as Tundergarth in the census). I have attached images of both pages for you to download in case they are helpful, along with a page describing the scope of the parish survey to set these in context."