AP Racing disc cracked

[DuncanR]

Lent one of my rears to a mate after he got a puncture at Bedford trackday, (would do the same if any of you chaps are at Bedford and get in trouble, I only live down the road , also if you need a trailer ride home hopefully not, give me a call mob 07768 633717) got it back this morning and upon removing wheel to swap over, noticed a crack on the inner plate of the disc. This had manifested itself in the form of a rotational clicking noise, caused by a rough area of the outer disc plate (see pic)
temporarily picking up the pad as the wheel turned, banging it against the top of the caliper housing and dropping it again. As you can see from the Vernier reading this disc has only got approx. 1 mm of wear on it from its original thickness of 20mm.
The disc I believe is a CP4475-122G8 as the brake kit is a CP5144-1004.G8. There is no evidence of any thermal cracking on the surface of the disc. I know its difficult to estimate how long discs will last as a result of how many track/ring days it has done, but would anyone hazard a guess? Me thinks a visit to Thorney is on the horizon !