Just spoke to a fixings company on the phone, they sell a kit which includes the helicoils (15mm long), the tap and the insertion tool.

The depth of the thread in the block is around 25mm so this raised the issue of the 15mm helicoil being dwarfed by the 25mm depth of thread in the block. The company said they have 20mm long inserts for an extra few quid. Looks like I'll be paying them a visit tomorrow to relieve myself of £35

Fucking car.