I've got an iphone 5c, and someone has texted me from a different phone today a message, which from their phone had to be sent in 3 parts. Only parts 1 and 3 have come through, and there is some important stuff from the second part that i need for this weekend.

Is there any way i can retrieve part two of the message without asking them to send again?

This happened to me before, you can't retreive the message you just have to wait for the other part to come through, mine was a couple of hours later.

You could try airplane mode and leave a few minutes to see if that forces it through.

You could try manually connecting to another network - your phone won't latch on, of course, but there's something about the process of de-registering with your network and then registering on again that can sometimes help budge SMS messages stuck in the system to emerge (it might well depend on the kit used by the network).