I have a W Reg Ford Focus which I've been having problems with.

Every now and again the engine light will come on and it'll lose power (But doesn't cut out)

Just checked the error codes and its shown 9318.
Searching the code comes up with "Battery Voltage Low"
Would this battery problem cause the slow down?

 

Thanks for any replies.

Suggest as a starting point you get a multi-meter and check the battery voltage at rest, while running and at rest again a while after. This will help tell if maybe its the alternator not charging the battery, or the battery not holding the charge. There are a few you-tube videos on how to do these checks. A year or so ago I had a dodgy battery and it wasnt simply about starting, it started fine, but lack of proper power while running made the ecu and transmission misbehave and it cut out too. It had given me all kinds of alarming expensive sounding codes, but it was just the battery in the end.