Lumpy Idling/ hunting/ feeling flat

Hi all, new to the forum,

Just wondered if anyone else has this problem with a 2012 focus diesel 1.6.

on start up the idling is crap! Lumpy, hunting, when you press the accelerator, it’s not a strong rev if you know what I mean, it feels like it’s spluttering into life. As I pull off the drive it’s very jerky. As the car warms up it gets better. 
I thought about the fuelling, injectors, I’m not mechanically minded at all, maybe idle valve .... then I had a new battery fitted, and the car was like new, positive strong idle, no lumpiness, pulled like a train, no  spluttering. Great I thought, problem solved... this lasted about a week then back to normal. So would it be something causing the battery to weaken, so the car hasn’t got enough oomph from the battery??? I only do short journeys to work. I have to ride the clutch or over Rev the engine so that it pulls away, without kangarooing, when cold. Look forward to you lads solving my frustrating problem.

ps, I have the car serviced every year, the garage just keep saying, yeah the fuel filter!! Don’t solve it, so rather than spending £££’s on guessing what the issue is, I was hoping someone on here could help..... thanks 🙏🏻👍🏻