I find it rather bizzare that you need such long spikes to aerate what is effectively a bit of lawn.
Now, I'm no gardener, but most of the aerater implements I've ever seen (that are pulled behind tractors) only have 100mm (4") spikes on them.
Even so, in the underground cables I've done in the past, you can't go past a few thin planks of treated timber over the top of the cable, irregardless of wether it is neutral-screened or armoured, these cables are made to be buried direct for those that are wondering.
Anything less should be in conduit anyway.
Let's face it, these days if you're not young, you're old - Red Green