Spoke with British Airways EC this morning,
I've underpaid the tax on the initial booking, as per them - they will honour original flights at current cost.
Changing first leg to depart MEX they will recalculate tax, and thus charged £125.50, plus as expected £35 change fee, as first explained in the thread; quoting figures from the standalone flight calculation got me nowhere unfortunately [MENTION=51151]Strawb[/MENTION], they were empathetic but alas.

i obviously could split the bookings in MIA but I don't want the hassle of sorting a self-trasnfer in MIA, and above all if flights get delayed out of MEX, I'd rather have some chance of being rerouted by BA.

I'm resigned to paying the 'full' amount at this point, but begs the question, how are British Airways calculating this if its not a simple tax flight 1 + tax flight 2 as suggested by Strawb ?