Unity3D app crashes out after error "Warning -> ApplicationDidReceiveMemoryWarning'

Not sure what the cause of this is, I thought it might be memory running out but attempting to limit the amount of memory used hasn’t helped at all, any ideas?

Thats not a crash.
When iOS sends you this warning 3 times in a row, then iOS will kill your application for using more memory than available.
When you get that messages, you are meant to respond to it and clear up some memory.

As a rule of thumbs regarding the RAM + VRAM you can use at a time on the devices:

On 3GS, iPad1, iPod Touch 4 you have 100-120mb for RAM + VRAM.

On iPhone 4, iPhone 4S, iPhone 5, iPod Touch 4 and iPad 2 / 3 you have 250-300mb of RAM + VRAM

On the iPad 4 you have > 500mb of RAM normally.