Neck and shoulder discomfort I have a Raleigh Strada hybrid with flat handlebars.

Have recently been getting some discomfort in the neck and shoulder area which I suspect is down to poor posture on the bike.

Are there any excercises I can do to improve things?
Possibly your position on the bike is putting too much weight on your hands, or making you lean a little too far forward than you want. Moving the handlebar up and back may help. You may want to check that your saddle is placed correctly. If it's too far forward it reduces your ability to support you body with your legs as much which places more weight on your arms and hands.

You can also try moving your hands into different positions while riding to avoid aches from staying in one position for a long time. 

You can try neck and upper body stretching exercises, they might help and it's unlikely they'll do you any harm.