So I washed the whole thing in sepia, managed to avoid most of the defects in the crevices and I ended up with a decent base to start with. You'll also notes I did the red and white quartered heraldry on the front, both left and right and even got the highlights done on the relief sculpture.
There's still some washes to do on the tracks, weapons and the red elements.
It's a bit one dimensional in places, I saw an amazing Deathwing Land Raider a while back with a massive Deathwing symbol diagonally offset over the rear and sides . I really wish I'd done something like that but I'll persevere with what I've got.
I'm still highly wary of the next step, but this is a much more comfortable starting point. I need to not think to hard about it as it doesn't matter in the long run. I just wanted a painted tank, it doesn't need to be perfect so just need to crack on with it!