It sounds as if you're actually paying into the RM Defined Contribution Plan with Scottish Widows.
If you leave RM you won't be able to continue paying into it directly, but you can transfer it to another pension provider and then pay into via them. Or else keep it where it is until you decide to take it.
Full details are in the plan guide, available to view here: https://www.scottishwidows.co.uk/save/royalmaildcplan/
along with other info you might find useful.