Jack, strong smell of rat there. If your pension provider is deducting tax at source, they must only do it once on the same money.
To put it another way, if they are calling back money on which you have already paid tax (=had deducted), they should have credited the tax deducted on the clawback against that deductible on your current pension payments. (Any tax liability on the clawback, presumably, now devolves to your ex and is not now your problem).
I can't be sure without seeing the numbers but if this is the case, possibly the numpties at your provider are not quite up to the job, yet could perhaps sort things properly and fairly easily given a little friendly enlightenment from your end.
Otherwise, you can ultimately claim any overpaid tax back fom HMRC, but that is more paperwork and (I say again) if you have good records to hand that is a huge advantage in sorting things.
Please sing out if you need a hand on this one, per PM.