Does the SCX(n) cylinder have an internal expansion vessel?

Yes, the cylinder has an internal air bubble. For more information on recharging this, please see the below link. 

https://glendimplexservice.freshdesk.com/support/solutions/articles/79000137343-scx-scxn-cylinders-water-can-be-seen-running-through-the-tundish

Applicable Models:

SCx Range - SCx130i SCx150d SCx175d SCx175sd SCx175si SCx215d SCx215i SCx215sd SCx215Ssi SCx255sd SCx255si SCx305sd SCx305si

SCxn Range - SCxn130d SCxn130i SCxn150d SCxn150i SCxn175d SCxn175i SCxn215d SCxn215i SCxn255d SCxn255i SCxn305d SCxn305i SCxn80d SCxn80i