For anybody interested, the 507/8s are a sub-class of EMU that included the 313, 314 and 315s. In total, more than 750 units were produced, mostly in the 70s, but very few of them are still in use today (for example, 20 out of 64 units for the 313 are still active, according to Wikipedia).
Merseyrail has 31 507s and 25 508s, so as has been said, having 25 units out of service is a very big problem for the network. Spare parts aren't exactly rolling around in buckets for trains over 40 years old.