Regulation C6 Right to opt in for Members of the House of Commons
An opted-out Member may apply to rejoin the principal scheme as a participating Member as from the date of any General Election or by-election at which he was elected to membership of the House of Commons (“the effective opt-in date”) by giving notice in writing to the Trustees within a period of three months beginning with the effective opt-in date provided that—
he pays to the Trustees within twenty-eight days of their acceptance of his application, or within such longer period as the Trustees may in their discretion determine, the sum certified by the Trustees as being the amount which would have been deducted from his salary under regulation D1 between the effective opt-in date and the first date thereafter when a deduction from his salary under regulation D1 is made; and
at the effective opt-in date, at least three months had elapsed since he was last elected to membership of the House of Commons.