Trouble with YAML Transformer

[Graham Ashby]

What I want is an attribute that I can include in a template. 
I'm trying to get the host.name of the first entity in the cluster, so 
what I've got is:

    brooklyn.enrichers:
      - type: org.apache.brooklyn.enricher.stock.Transformer
        brooklyn.config:
          enricher.sourceSensor: $brooklyn:sensor("cluster.first.entity")
          enricher.targetSensor: $brooklyn:sensor("cluster_first_host")
          enricher.targetValue: 
$brooklyn.sensor("org.apache.brooklyn.api.entity.Entity", 
"cluster.first.entity").attributeWhenReady("host.name")

But I get the literal value 
"$brooklyn.sensor("org.apache.brooklyn.api.entity.Entity", 
"cluster.first.entity").attributeWhenReady("host.name")" 

So, what am I doing wrong?

Thanks
Graham