Not on the Defender yet but I fitted Cross drilled and slotted discs on my RRC to all 4 wheels when I had it and it made a lot of difference. When I can afford it will be doing the Defender.
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I'm sick of my pads squealing on my D2a I'm going to try Akebono ceramic pads next time, apparently less noise and dust. See link Disc Brake PAD Euro Ultra Premium Ceramic Pads Rear Front Akebono EUR493 | eBay
I had a similar problem with my brake pads. I ended up getting the discs machined and the problem went away. The guy at the mechanics seemed to think that it is normal if you have worn discs?