The handbrake on my fusion is very poor, I have to really pull on it to stop it rolling on hills. Surprisingly it's never been an issue for the MOT.

I removed the drums and gave them a good clean with some brake cleaner but it has made no difference.

Any ideas how to diagnose cable problems? Where to start?

The shoes are probably glazed, rough them up with a bit of sand paper.  Then manually adjust them so there's almost no slack in the system.  Then finally tension the cables from inside.

Tbh though, if it's passed MOT it's probably about as good as it'll get, I never found the drum handbrake on Mk6 Fiestas particularly good either.

Some people adjust up the handbrake cable to compensate for the auto adjusters in the rear drums being stuck and not doing their job. Of course it is easier to adjust up the cable.

On my mk6.5 fiesta I took the drums off to do the job properly (adjusting the auto adjusters to correct point manually). This did make the handbrake noticeably better than it had been when just doing the cable adjuster.   It's all to do with the angle of the levers in the drum compared to the fulcrum point of the levers in the drum.

By the way, on mk6.5  fiesta the best way to remove the drum is the four bolts on back of brakeplate, not the centre bearing nut. I assume fusion brake is same but don't know for sure.

I ended up removing the drums and manually adjusting the auto adjusters, on the passanger side the drum was far from the shoes. Ands the shoes had a lot more material on them. I also noticed that the passanger side cable was loose under the chassis. I have know secured it to prevent it moving.

The handbrake is a lot better than it was before but it's still not fantastic.