Complex sentence: The party is strongly critical of mainstream parties for what it views as a failure to safeguard the Act of Union 1707; this criticism extends even to the Scottish Conservative and Unionist Party, who they believe are not being'unionist' enough.
Split into simpler sentences: The party is strongly critical of mainstream parties that they view as failing to safeguard the Act of Union 1707. This criticism extends even to the Scottish Conservative and Unionist Party, who they believe are not'unionist' enough.