The Bollandist fathers in Belgium have maintained a wonderful database of the medieval manuscripts containing copies of material about the saints, especially their “Lives” and this has been fed into the new Legendiers Latins website. But the information is not comprehensive. For instance, for St Botulf, it does not contain any mention of British Library manuscript Cotton Tiberius D. iii.
I don’t know much about the Cotton manuscripts. As so often with major manuscript libraries, the “Cotton” collection is so called because it was assembled by an individual, whose manuscripts came into the British Library in a bunch. In this case the donor was Sir Robert Cotton, or rather his grandson, and the circumstances may be read at Wikipedia here.
Cotton divided his manuscripts into groups, which he named after Roman emperors. Apparently each group was in a particular book case, with the bust of the emperor on the top. So the shelfmark tells us that this manusccript could be found in the “Tiberius” bookcase shelf D, number 3.
Unfortunately the Cotton manuscripts were all damaged in a fire in the 18th century. Some were preserved intact; others burned to a crisp; and everything in between. Scholars still needed to be able to consult the remains, so ingenious solutions were found such as this:

The British Library has a webpage which has links to digitised copies of the manuscript catalogues. There are two catalogues for the Cotton manuscripts, one from 1696, one from 1802. Oddly the newer catalogue is less comprehensive. Here is the entry for our MS:
The Smith catalogue entry is:
Entry 53 is our text, and tells us that it contains the letter “ad Walchelmum episcopum”, i.e. the dedicatory letter to Wakelin, bishop of Winchester. There’s no folio numbers, but Hardy’s “Descriptive catalogue of materials” tells us that it’s folios 223v-225v, and 13th century.
The truth is that looking at the Smith catalogue is an overwhelming experience for anyone interested in the history of our people. This is a vast collection of material, all of it of the highest importance for English history. Cotton even owned a Magna Carta! He collected all this stuff from the ruined monasteries. It’s one thing to read words about how important the collection was for historical purposes. It’s quite another to read through the list of saints – all English or British – and realise that this is the raw stuff of medieval England.
This leads us to the next question – what survives of BL Cotton Tiberius D. iii?
I don’t know the answer. I do know that back in 1901 when Horstmann issued a new edition of the Nova Legenda Anglie of Capgrave, originally printed in the 15th century by Wynkyn de Worde, he collated the “Life” of St Botulf with the Cotton manuscript. There are variants in the footnotes! So it must be readable to at least some extent?
I do have a readers’ card for the British Library manuscripts department. Sadly it is no longer valid since the cyber-attack a few years ago. The BL website informs me that I would have to get a new letter of introduction from an academic in order to get another. Curiously I must get a fresh one every time I renewed the readers’ card (!). Getting a new letter of introduction is awkward for an independent researcher like myself. Indeed I don’t quite know whom I would ask. I don’t really want the journey anyway.
I will pop an email over and see if there is another way!