Posted: 04/08/2013 at 06:42
When the blooms on that stem have gone over, I would cut it back by half to an outward facing bud. In the spring(late Feb/early March, depending on conditions) I would prune the weaker stems back hard, down to just 3 or 4 buds, pruning to an outward facing bud; leave the stronger stem alone. As pruning promotes growth, that should help the bush even itself out a bit.
After pruning give a mulch of well rotted farmyard manure (don't let it touch the stems) or a dressing of Fish, blood and bone and make sure your rose bush has space to grow - it's looking a bit overcrowded, which won't help the weaker stems.