We ended up buying GTA5, because I couldn't resist, and it needed more than the 8GB storage even when the PS3 was empty, so I grabbed my old hard disk and set about opening up the PS3. The CECH-400X PS3s are an absolute bitch to get apart! Try as I might, I couldn't find any instructions on Sony's website - it turns out I must have been using the wrong search terms because they're found in the instructions to the official hard disk caddy. I sat the hard disk in there and proceeded to enjoy GTA5, and ordered a cheap caddy from Hong Kong via Ebay.
It arrived today, so I put it in. Sabriena opened up the package and it turns out it's not a cheap caddy - it's the actual Sony unit #CECH-ZCD1 (though I suppose it's not outside the realm of possibility it's counterfeit). It didn't come with instructions for some reason, but it's pretty self-explanatory. For a measly five bucks, the quality of the unit is pretty impressive.
So the PS3 is all put back together now and I don't have to worry about breaking an SATA connector because I bumped the entertainment centre with my hip.