The main FHIR site has XSD's which you can import using studio. They may need a few tweaks on the way in but nothing major (if I remember correctly it doesn't like narrative)

From these you can build resources which confirm to Care Connect profiles. 

I'm not aware of any NHS guidance on FHIR Messaging except for Transfer of Care but I'd recommend this presentation on FHIR Messaging 

Simone Heckmann’s presentation  https://vimeopro.com/user12740828/hl7-fhir-developer-days-2015-amsterdam/video/171535590

Amendment.

It seems a better way of getting metric/audit out of Ensemble for splunk would be to use splunk sdk for java or use a REST API. Which one to use depends on effort but our move to FHIR (across the enterprise) means we should be looking at FHIR Audit.

[In practical terms: audit a HL7v2 ADT_A28 message in Healthshare Connect FHIR registry as a Patient POST or an ADT_A04 ED as Encounter ED POST. We'd expose that FHIR Audit to splunk via a Splunk REST API or Splunk Java SDK.]