criminal convictions

[dog breath]

if your conviction is spent do you have to declare it on visa applications?

[Buenchico]

The Rehabilitation of Offenders Act only applies to circumstances covered by UK law. Via applications are covered by the laws of the relevant foreign countries, so the ROA doesn't apply and you're meant to declare all convictions. For example, see this page relating to visas for entry to the USA:
http://www.usembassy.org.uk/cons_new/visa/niv/ add_crime.html

Chris