Hi Matt
It sounds very much to me like your seat frame is cracked. I have the same problem, but have not done anything else about it to date. It is a known issue. The seat moves as if it isn't locked, usually when negotiating a right turn on mine. My seat apparently has a cracked weld on the seatbelt side. The starlock washers had also fallen off , which I fixed, as detailed earlier in this thread.
The dealer immediately knew what I was talking about, but as Mazda had not issued a service bulletin, it would be about £400 to fix. This involved ordering a new frame and labour to fit it.
If the car is within a Mazda warranty, then I would have thought it should be fixed FOC. You could always print off this thread and take it to the dealer as some backup to your request for repair. The kit you've linked to I don't believe is going to be any use to you.
Hope that helps.