While the spider FX was cool, I thought it was lame that they were really bacteria. Giant bacteria should look like giant bacteria. Giant spiders should really be giant spiders. It's like the FX department came up with one idea and the writing department wrote something else, and they just rolled with it.
I'm going to guess that they had the VFX for giant spiders lying around, and re-purposed it. Standard BBC procedure, like a nod to the old days, really
