2004 Fiat Ducato starting problem?

My husbands FIAT Ducato 2.0 (petrol) van is throwing a wobbly!
Last week, battery was replaced (at home) due to failure to start (slow turning over)
Starting was still bad. My husband investigated the 'stepper motor / slow idle motor'. After testing it (as demonstrated on Youtube), it would no longer idle but immediately revved strongly!
This was replaced today but still the problem persists.
Does anyone have any ideas where to look next please?