Ask the doctor for a fit note which recommends a phased return to work Tinks. I have a colleague in a similar position to you who has been on half shifts since Christmas and has recently been given another fit note for the same level of shifts for the next 3 months. You might not need the same level of phased return or be able to afford to be on reduced hours, but it...
Big hugs tinks - it's not a nice time, but you can do it when you're ready - don't let them push you until you ARE fit to make a phased return. Slining you back on to consecutive night shifts isnt reasonable IMO, given why you've been off.