The Canterbury Charm

The runes were added after the manuscript was complete, sometime before the year 1100ad.

What make the runes so fascinating is they pray to Thor. Which has lead some to argue Thor was still being worshipped in England in the year 1100ad.

An alternative suggestion notes the charm was written in one of the most important monasteries in England, access to the book beyond monks and visiting dignitaries would be unlikey.  And the book is an ecclesiastical text, probably only interesting to people in the church. 

This had lead people to believe it may not have been a pagan, and instead Thor was still a suitable figure to be included in Christian magic spells at the time.

That the runes are in Norse not English is another mystery.

A Danish monk being at the monastery is one explanation, another is an English monk born from the area of the Danelaw, fluent in Danish. Or even an English monk of mixed English/Danish parents. 

I have also seen an explanation that one of the monks may have been ill, and all English charms failing they tried foriegn one.

Whatever the explanation, in a major Christian monastry in England, a pagan spell to Thor was written in the late 11th century and this seems to have been ok.