Films of Liverpool Crematorium records are held in the Liverpool Archives at the Central Library. This is almost certainly where Ancestry were given access to them to put on the website. According to the Archives' catalogue, it appears they hold the original registers as well. If there are gaps in the records available on Ancestry, the explanation is likely to be that Ancestry did not upload all the films. This is what I was told when I was looking for a marriage in St. Peter's, Church Street which took place in the 1880s. It appears the staff had told Ancestry about it on multiple occasions. On Ancestry, there is a big gap in the films of marriage registers for this church which covers a period which starts in the1850s and does not end untiil about 1901. All the films are available if you can visit the archives, but I'm afraid it's frustrating for us all at the moment.
