Inner wing / scuttle repair - how?

[Tomoose85]

I've found a bit of rust on my 1997 Mk1 1.8iS that I'm a bit concerned about, well, I knew about it when buying it, but ho hum.

Its a very small area, seems to be on a seam between the inner wing and the scuttle area.

How would one go about repairing this? I hope the photo makes it clear.

Any idea how much I should expect to pay to repair it? I intend to keep the car forever so it would need doing to a high standard!!