Mystery caterpillar

Can anyone identify this little chap?

One of my clients asked me to have a look at some bedding she had in her front garden, which she thought had been clobbered by slugs. The odd thing was, it was mostly pelargoniums, which in my experience aren't that tasty to slugs so generally don't suffer much damage.

It was pretty clear as soon as I looked at them that this wasn't slug damage - no slime marks, and the holes were large and in the centre of the leaf - not like slugs at all, which usually attack the edges of leaves first and then if they are going to eat the centre of the leaf, they kind of scrape away a thin layer to begin with rather than eat it straight through all at once.

Anyway - it was all screaming caterpillar to me, so I hunted around for a bit and sure enough this is what I came up with. Trouble is, I have no idea what it might be - I'm not exactly an entomologist and my garden experience with caterpillars is limited to gooseberry sawfly and cabbage white butterfly caterpillars, both of which I can identify at a hundred paces and despatch accordingly.

I'm a bit concerned though that this little chap might be something interesting. I had a look on the pragmatically-named What's This Caterpillar website but despite a happy hour browsing their gorgeously illustrated plates couldn't come up with a conclusive ident - the closest I got was the unfeasibly rare Orache Moth.

Thinking it rather unlikely that we've turned up the kind of thing you send to the Natural History Museum, I sent these pics in to Wisley's advice centre to see what their bug people can come up with. But if there's anyone out there who's looking at this and thinking to themselves, "Doesn't she realise that's a pelargonium leaf-stripper?" please don't hesitate to enlighten me! Until then, we're holding off the spraying/squashing/nematode treatments in the assumption that it's innocent until proven guilty.