Sorry for the delay in responding to your message.
Can you clarify what you mean by 'report showing NHS last 18 months'. Are you looking for the number of patients seen over the last 18 months? Or the amount of time in the diary that was booked for NHS patients? Or the amount of time that was spent in the surgery for NHS patients?
If you can clarify what you are looking for then we can see if it is possible in R4.