Rotate Flat (FITS file) - Help Needed!

[Stub Mandrel]

Last night I was imaging with the scope flipped for only the second time ever (I have a new location biased west). I thought I would be clever and rotate the image in Sharpcap so everything would be the right way up.

What I didn't realise is this would mean my flats (taken without rotation) would be the wrong way up resulting in uneven results and a gradient.

I can't see a way to tell Deep Sky Stacker to rotate the master flat or a way to save a rotated but otherwise unedited flat in FITS liberator.

Can anyone offer a solution?

[Stub Mandrel]

Update: excessive noise reveals Ii need to rotate the darks too, of course, and my dark flats.

I've found Irfan view can do this and save as a TIFF, which will do BUT it insists on stretching some FITS files - making them useless for calibration (but not others?)