Hello again and thank you all for all your good wishes and words of support.
I had my weekly meeting with the person responsible for HR at work and I told her how exhausted I was feeling she suggested I do less - so I am planning to do mornings - Mon/Weds/Fri next week and see how I feel and she suggested reducing it further if by Wednesday I was feeling tired. I am very lucky that I work in the public sector and, like you say, have generous sick pay etc.
I think going back to work has been good for me in the sense I get to see people and it helps me to feel a bit more "normal" but I appreciate what you are saying and I will take it easy outside of work and rest as much as possible. As I said I was feeling right as rain until I went back to work - I guess I had lost track of how much I was resting when I was off work.
Thank you all again x