Hi everyone, I will try and keep this as short and clear as possible but the nature of non party list seats has complicated things!

Independent/Independent Grouping elected constituency MPs:

A simple one to start, if someone is elected as an Independent/Independent Grouping MP representing a constituency, they will always be the holder of that seat, regardless of if they join a party. This means that when they resign, there is a by election.

Party constituency or list MPs:

These always belong to the party. If the party dissolves, the MPs become Independents, but if they join a new party, the seat then becomes permanent property of that party, as if it had always belonged to them.

Independent/Independent Grouping elected list MPs:

If someone is elected as an Independent MP representing a list seat, they will be the holder of that seat only while they remain an Independent. If they join a party, the seat will then become on loan to the party and they can replace the MP if the one elected resigns. However if the MP, whether the original one or a replacement chosen by the party, leaves, they take the seat with them. This means that if they are Independent either through being elected as such or through being elected under an Independent Grouping, the seat will remain vacant until the next general election if they resign and they are not members of a party or independent grouping at the time.

This is the most complicated part and the bit that is being debated at the moment.

Because of this, the Progress MP, who was elected as list MP for an Independent Grouping, but has resigned whilst a member of the Solidarity Party means that the seat remains on loan to that party and they can choose a replacement, so there no by election. The SNP MP was elected as a constituency MP for an Independent Grouping, and as such there is a by election, because the seat always belongs to the person elected.