My black tri's coat is going a reddy colour on the rump no where else any suggestions to try and avoid this happening or help to retain the black.

age can do it as can sun my solid black gsd had a hot spot on his side and the hair has grown back with a slight browny red colour. It isn't full back yet and I amhoping the top coat will be black. It looks like he has gone rusty lol . Try shampoo for black dogs

Dead coat will go rusty, have you tried grooming it out, it might just not be ready to come out yet but dying.  I tend to find the moult starts from the rear end moving to the front of the dog over a couple of weeks.

He is bathed and groomed regular for showing and I give a quick brush everyday coming back from the fields. What black shampoo do you recommend.

Not much you can do for dead coat other than wait for it to shed.

I use gallop one of the equine ones in between being professionally groomed. She is poo roller yuk !