Amending HTTP Header for embedded resources

Hi

I am using JMeter for loading a system under test that is monitored by an
APM solution. This uses an HTTP header to identify webpages or web service
requests.

I currently have an HTTP Request sampler with retrieve all embedded
resources enabled and a child
HTTP Header Manager defining the header. Unfortunately this results in the
main request and all embedded resource requests having this header, with
the APM then aggregating response times across all of these.

Ideally I am looking for the main requests to be aggregated separately to
the embedded resources and so would like to set the header to a different
value.

I cannot see a way to do this with the current components, does anyone know
the best way to approach this. Would I need to look at extending the HTTP
Request sampler?

thanks

Dan