Still a lot of money, and only 2ft long, as it is not an expensive rod, two part whole cane with a split cane tip spliced in I don't want to spend a lot of money so, I am cheating today, bought some shrink tube for £4 off ebay, arrived this morning and going to cover it with that, not ideal but if I really like the rod I will then consider replacing with some nice cork but it is not a top notch rod, just something I think I might like using in the future.
I experimented first with a cheese grater, some old wine corks and filling the residue into the bigger gaps with super glue and then sanding down, not really happy with the result, finer dust might have been better as Seth suggested, anyway going to cover with shrink tube now, whip some rings on and see how it goes.
Still think you have made some good points about the filler, most of us would never think to look before, well, certainly not me so I will keep an eye out for it if I buy a new rod.