Posted: 14/07/2014 at 06:55
I'm a little bewildered - you say that you've bought a new rootstock - a rootstock is what the named variety is usually grafted on to. Have you bought a rootstock or a grafted variety?
Working on the premise that you have a grafted variety, my thoughts are these:
Firstly, all stone fruits should be pruned in May/June, to prevent them contracting Silverleaf disease. They should never be pruned when dormant.
Secondly, the thing to bear in mind when pruning is the old saying 'growth follows the knife'. That means that you should prune hard to encourage growth - so the side that you want to grow the most is the side that you should cut back most severely. The other side you should leave alone or just tip prune - eventually the two sides will even up.
Lots of really useful information here http://www.readsnursery.co.uk/pages/Apricot-Trees.html
Also, I can't see the ground around your tree clearly, but I get the impression that it's a little overgrown - you're probably going to do this anyway, but the tree needs a circle of clear ground diameter 1 metre, to enable proper watering, feeding and mulching - otherwise grass or other plants will compete.