Generally speaking a split has no effect on your basis, unless some CIL is received in which case you can assign some basis to the "sale" of the fractional share.
Unless there's some sort of "income" or "cost" effect reported in your income tax return associated with a rounding up or down, in the absence of CIL I'd think your original basis simply carries over to to you new holdings.
(I must admit I've never encountered a "no CIL/round up or down situation. Company involved?)
Tom Young
