I've got one step further. I can now see how to create a note but part of the custom action is to update the note with the below info but the issue I have is its generating an error - Error: Formula is incomplete
£{InvoiceAmount} requested to be invoiced against {$Project.OrderID} with the following description:
{Description}
Milestone Name: {$Name}
Project Name: {$Project.Name}
SOR ID: {$Project.OrderID}
Invoice Due: £{InvoiceAmount}
Actual Effort: {$ActualEffort}