[susan53]

I've never seen an explanation that holds up in terms of meaning difference when you're talking about excluding a specific instance from a general statement - eg :

I know nothing about it except/except for what you've told me
Everyone was dressed formally except/ except for John.

- for me they're more or less interchangeable.

However, only except (or rather except that) can be followed by a clause expressing a reason :

We would have gone out except (that) it was raining.