Thanks, I did try this but was receiving errors. This was the syntax I was using in the DTL:

..Piece(source.{AlternateVisitID().ID},"~",,*)

This is the full PV1 segment:

PV1|1|O|NPID15T^MEDIFREN^^^^^^^NURSE LED "TB" TELEPHONE CLINIC|||||||||||||||1|015728690002||||||||||||||||||||||||||20210203114600|||||15081942~15081951~15108961~15280550

Thanks for the help Enrico. It's working perfectly now.

Martin

Thanks Enrico, 

I've written the custom function...

ClassMethod PostCodeLastCharacter(PID11 As %String)
{
Set ReturnValue = 0

if ($EXTRACT(PID11,*) = "Z")

{set ReturnValue = 1}

Quit ReturnValue

}

I've added a trace to my rule which returns the relevant 1 or 0 (depending on the post code). However, my rule condition which references the function doesn't trigger an message...

PostCodeLastCharacter(HL7.{PID:11(1).5}=1)

Have I got the syntax correct?