This tutorial may or may not solve the issue for you as there could be other factors causing it but this fix will allow you to atleast get the door open and fix where the deadlock is seized in the locked position.
This is worth a shot rather then paying £400 to replace the locks that may not be nessery.
I also recommend you apply lubricant to any other locks on the doors at the same time especially inside the lock (via the panal) on the rear doors even if they aren't stuck.
![](https://s2.save4k.ru/pic/-TjMw6ORydA/maxresdefault.jpg)