RegExp not selecting the FullName item I expected

I suggest you read through Understanding Web Object Identification and Object Models. 

Note, that FindElementByAccessibilityID method returns the first found object that matches the condition in the tree structure of Process("uk.co.rpssoftware.designer.anglian")

You need to get the correct Parent for FindElementByAccessibilityID to work.

Also, I think your coding is incorrect, as your Find method looks like this (if you remove all the other stuff),

target = parentLevel.Find("FullName", "regexp:FindElementByAccessibilityId\(\"Cornice\\n\"\)*", 20000);

Try something like this,

target = parentLevel.Find("FullName", "Cornice\\n", 100);