Hi Jon,
I have both early and late class 47s and apart from the class 57 loops on the early bogies there isn't much difference.
The brake linkage missing on each corner of the Bachmann bogies comes in the detail pack, there should be 8 but my early class 47s came with only 4. They are a sort of y shape and certainly improve the look of the bogie when fitted, though they aren't the best moulding. Pete Harvey etched steps might be worth looking at too.
I also extended the fine pipework running along the top of one side of each bogie as it stops to short at the ends. I think the early bogies had this both sides, so I removed it.
If you want a few pics to explain things better I will wack a couple on my layout thread.
Cheers Peter.