If you had to do just 2 exercises, twice a week in order to get an all round core of steel... What would they be?

Two that I find great for strength and injur prevention.

First, back extensions, lie on your stomach with arms forward and lift both simultaneously.

Second, think they are technically known as reverse crunches or gorilla press. Lie on your back, draw your legs up to your chest keeping your back on the floor.

These two have improved my core strength and got rid of most of my back problems.

Other great ones...

Plank- press-up position, but on elbows/arms, and just hold it for about a minute

Leg raises.

Bryn

Sideways 'plank' is even better.
Left elbow and forearm on floor. Right foot on top of left, raise your hips (body is now in a straight line. Sideways).
you can hold your right arm in the air to keep balance, but better if along your hip/leg.

When you can't hold it any longer change sides. Repete five times each side.

Don't do this if you have knee or ankle problems.

yep, sideways plank is pretty useful, when I do that, by combining it with leg raise you can get a really great work-out.