Strange growth on Blackthorn trunk

Should I be alarmed by what looks like a very large scale insect on the trunk of my Blackthorn. I'm sure it wasn't there before and it's the only bit affected.

It is hard and will not be dislodged by poking at it.





Any suggestions?

Maybe

Black knot, caused by the fungus
Apiosporina morbosa (syn. Dibotryon
morbosum), is a common canker disease of
many trees in the genus Prunus. On susceptible
species, infection by Apiosporina results in
the formation of black, spindle-shaped galls on
branches that reduce the value of trees for fruit
and lumber and ruin the aesthetic value of landscape
trees. Hosts such as almond, apricot,
blackthorn, most cherries, chokecherry, peach,
and many wild and cultivated plums are commonly
affected. The disease occurs throughout
the United States and Canada.

And Scotland? But Billy's right. It is a fungal gall or fruiting body of some sort and not a "critter."

If you don't have a agricultural center of some sort near you that can help with a specific ID and suggest a treatment, maybe Google can help -- especially if you limit the disease search to the UK.

My initial thought was a canker of some sort and subsequent searches have given examples that look similar. Am hoping one/some of the Blackthorn owning Brits might have encountered it before.