This might be a reasonably small piece for the price but I cannot begin to describe to you how rare a find this piece is. Agatized Coral fossils just don't happen in the UK. I read an article when researching if I thought I'd found what I thought I did. The article said these are incredibly rare, to the point that it was probably something else. Having seen and found fossilized coral from Scotland, I recognise the formation. 
This is a piece of coral that resembles the burrows of small creatures. You find it in East Lothian but this piece came from Ayrshire. You don't find Agatized Coral in Lothian. 
I am so stoked with this find that I am keeping the other half in my personal collection. If you buy this we will be twinses. Lol.