Did you take the 180 deg shift into your account ?
Since your measurement was done when the loop was closed, there must be an additional 180 deg phase shift (in other words, minus sign).
In the end I just fitted the response magnitude. I was initially fitting the complex response function, but ran into problems which I think were cased by overall phase offsets between the data and test function. Can I canvass for opinion if fitting the magnitude is OK, or should I try again fitting the phase too?
I thought I had, but apparently not, and I'd made another error or two in the complex version of my fitting routine. I've fixed them now, thanks! I'll put up the new fitting results tomorrow morning.