This request is almost the same as your message in look-up requests.
I have only ever searched two films of Thornbury records and that was four years ago. I was interested in the early 19th C and, if my memory serves me right, the entries for the years in the parish register I tried to search, were awful and most were nigh on unreadable. I have never offered look-ups for Thornbury - but with my memory
You ask for marriage and death certificates - there were no such things until the Sept qtr of 1837.
The only way you just MIGHT find the info you want is by hiring and searching LDS film number 427790 which shows the Bishop's transcripts (BT's) written annually by the vicars/curates from the Thornbury parish church registers 1578 to 1812. The film was used by LDS members and others to produce batch number C057831 at the Hugh Wallis site http://freepages.genealogy.rootsweb.ancestry.com/~hughwallis/IGIBatchNumbers/CountyGloucester_(N-Z).htm#T
but burial info. is not given in batches as a rule. LDS batches do not necessarily give every baptism and marriage entry given in BT's because some may have been deleted after being used in LDS members' submissions.
BT's are expected to contain information concerning baptisms, marriages and burials but early ones are often difficult to read and the LDS library advises that many years are missing from the Thornbury ones. Then there can be a problem with Latin to say nothing about recognising the individual letters making up the words. I speak from experience of the 17th C entries in the Horsley film I have on hire and I've searched over thirty LDS films - some twice
I note that there are also microfiches available for hire - see the LDS catalogue at http://www.familysearch.org/eng/library/fhlcatalog/supermainframeset.asp?display=titledetails&titleno=702601&disp=Computer+printout+of+Thornbury%2C+Gloucs%20%20&columns=
With such early dates it is unlikely that any answer will be forthcoming so I advise that you contact your nearest LDS FHC but be prepared for disappointment if you hire a film.
P.S. Thornbury was in Avon and Avon is now defunct thank goodness