I think there are a lot of issues around this and wondering what is the best forum for discussing them? [I’m also not keen on doing all the work 
]. Here, https://chat.fhir.org/#narrow/stream/179189-uk, INTEROPen Ryver??
So far what I’m doing is starting from the HSCIC HL7v2 spec and converting it to a FHIR Message. I’ve gone for a message as it produces what we had before (and so brings in a lot of experience).
I’m finding this from HL7 International a real help https://docs.google.com/spreadsheets/d/1PaFYPSSq4oplTvw_4OgOn6h2Bs_CMvCAU9CqC4tPBgk