Judder - pulling (remaining) hair out

[Volvian]

I've reviewed other threads on this topic, but I have a couple of nuances that are confusing the hell out of me. I've got a UK 10-plate XC90 D5 Exec; a year ago I started to experience quite extreme judder when braking gently from high speed (c.60mph+). This was primarily felt through the steering wheel (not pedal) although the vibration passed through the whole car. Local garage diagnosed warped front discs and replaced them. Problem went away for about 6-weeks and then reoccurred (same symptoms). Back to garage and front discs replaced under warranty (bless them). Problem solved, but 6-weeks later I'm back at the same place. Garage has checked pads (fine), calipers and parking brakes (not sticking). The only fault they've detected is that the rear nearside disc is warped out of tolerance, and have advised replacement. I'll do this, but doesn't seem to explain why this problem has previously been temporarily alleviated by replacing the front discs (or why I'd feel the judder through the steering wheel)? Braking otherwise feels fine - responsive, no fade, etc. Thoughts, similar experiences, remedies welcome please!