To taper or not?

Company A takes over company B and Mr X receives qualifying corporate bonds (QCBs) in exchange for his shares. The held over gain is £100 000 but Mr X's shares qualify for 75% business asset taper relief so the post-taper gain is only £25 000.

Assuming that the shares were exchanged for the QCBs before April 2008 but that they are redeemed after 5 April 2008 (when taper relief will no longer exist) what will be the taxable gain?

One would normally compute the heldover gain net of taper relief at the time of the takeover and bring the £25 000 into tax at the time the QCBs are redeemed.

But is this correct or should taper relief be applied to the postponed gain only when it becomes chargeable?

If so the held over gain would be £100 000 i.e. before deducting taper relief and...