Does the recommendation letter have to be on a letterhead? Can the recommendation on a plain paper and signed but the email given is official email? I know the best thing is letterhead but is there any alternative?

1. Type of application: Tier 2 General Extension - Standard Service - Online application 2. If switching, category changing (switching) from: NA 3. Date of expiry of current leave: 13/02/2018 4. Number of dependants applying at same time: 3 5. Date application posted: 19/01/2018 6. Date application ...