How to fix instant lock, "unfortunately com.android.systemui has stopped" Moto G 1

I have has this problem for a couple of days, Normally a restart fixed it. However now am faced with the error message "unfortunately the process com.android.systemui has stopped." the first time I try and unlock after a restart. The phone has reset the background to the default home screen. which disappears when i unlock, going to a black screen however it still shows my apps and widgets, then a second or so later goes back to the unlock screen. I have tried clearing the cache partition multiple times. which has not worked, is there anyway I can fix this problem without a factory reset, or taking to a phone shop? Thanks Ben