Is there anyone out there who has had a draining seton put in (following partial fistulotomy) who can comment on how much blood/mucus drainage is normal?

I had my surgery and seton fitted nearly 4 months ago and it still drains a huge amount every day - the surgeon said after a month or so it would be much less than before the surgery, but it’s always been a lot more. About a month ago, I had increased drainage along with feeling very under the weather and a mild temperature - went to the GP who prescribed strong abx for an infection but it turned out no infection (at least not from the fistula) and the abx made me feel quite ill.

Fast forward to now and the drainage is more than ever - including a lot of blood - filling a dressing with a triple layer of gauze in a few hours - and quite painful. Is this normal? Any advice appreciated...!

I think that you should contact your surgeon as soon as possible. I'm certainly not a medical professional, but what you describe (especially based on what you were told to expect), does not sound at all normal to me.