I really like that pen, my only fountain pen that is not part of a desk set, and the nib broke today.
It looks like I'll be back in touch with Sam at Pendemonium again!
I bought a pre-assembled replacement nib, feed and section for my Waterman Expert from theonlinepencompany for 55 us (36 pounds).
It was nice just unscrewing the old section and screwing the new one in.
Is it vintage or modern. If more modern, replacement nibs, actually nib and section units are available from the Waterman/Parker repair center. Replacement for a Phileas cost me $30 shipped.