As a result of the collective smarts of the lovely people here, I decided to raise my t4 a touch and after about three days on a higher dose I finally had a good day yesterday.
From the moment I got up I was firing on all cylinders (or most of them anyway) and was active all day for the first time in ages. I hadn't had enough sleep really so I was a little tired, but it was a whole different kind of tiredness than hypo zombie tiredness. It didn't threaten to overwhelm me.
I don't know the mechanics of it, but it seems that trying to increase the proportion of t3 to t4, even by a small margin, was making me feel ill - sort of hypo and hyper all at once - so I will return to just a touch of t3.
I'm still on a much lower dose of meds than I was this time last year, so it may be that my thyroid woke up and tried to help which made me go a bit hyper, or it may just be that I will never feel well on more than a tiny touch of t3 and I need to again build up my t4 over time.
I'm going to ask the gp for a tft on Mon, so that should give me a picture of what's going on.
Apologies for forgetting who it was, but someone suggested that maybe after some time on t3 my body was able to process t4 better and maybe I didn't need the t3 (or not as much of it) to feel well now. That specific comment was especially helpful and led me to change things around, so many thanks. xx