Canon offer "fixed price" repairs in some circumstances. But there are restrictions:
- They'll only do it for some lenses, and I don't know for certain whether the 100-400L is one of them.
- They'll probably only offer a fixed price if it looks fairly straightforward.
The idea of the "fixed price" repairs is to make things faster, and cheaper on average; they'll take a quick look at the lens and offer to fix it for (say) £100. The alternative would be to strip it down to find out exactly what's wrong and give you a detailed estimate, but that takes time - and time is money. If you accept a fixed price estimate and it turns out that there's something seriously broken then they'll absorb the cost.
But I'd agree with HoppyUK that it shouldn't need servicing unless there's something clearly wrong. Lenses aren't like cars that need to be serviced every so often.