I contacted Motorola support who sent me this:
"Thank you for contacting Moto & Lenovo customer support.
Those messages are updates from your carrier, you have to accept those updates because are pushed by Lebara itself in order to add or fix network functionality to your phone."
Can't say I'm happy to just act on messages like these, especially when they come with a security warning, but I can't seem to find any other way of achieving the same result more securely. Maybe I'm being a bit over-cautious.