'Dust' Under the front glass - Sigma 17-50 2.8 (1 Viewer)

I've got some (what looks like a clump of dust) under the glass.



It's right in the centre of the lens, which cans sometimes be apparent in pictures.

I'd rather not send the lens off for 3 weeks, and I've been quoted £100 to get it sorted by a local camera shop.

How would I go about cleaning this lens by myself? whats the risks?

I've seen posts on how to do it on cannon lenses, but not this sigma!

Stripping the lens down is pretty straight forward, problems arise from dust particles in the air getting into or onto the glass once stripped out. I always strip lenses in the bathroom, seems to be the room with the least dust particles in the air.