I’ve got an 80mm stem on the scandal 29er,been out tonight and it was twitchy as ****, although part of that was down to the tyres being no good for the conditions (kenda karma, good on rocks, shite on mud). Will putting a longer stem on make it a bit more stable or do I need wider bars (currently 600mm wide)?

Neither will have any effect on the steering geometry – only on the weight distribution ( very slight) and feel. You may have more control with wider bars.

As TJ says but a longer stem will at least get more weight over the front.

A longer stem will make your steering less twitchy.

wider bars mean that you have to move your arms slightly more to steer as much,

so yes they could make it less twitchy.