Welcome to Goy Donmartin. It is growing long straggly branches instead of becoming strong and bushy by the sound of it. I would cut the flowering stems back to about 8" when it stops flowering. New shoots will emerge from that point (2 new branches). Next year do the same and eventually the plant will reach a height of 8'. It does not sound as if it needs feeding so do not give it any food. You may find you have been mis sold a different hypericum, not Hidcote, by the garden centre. If you can it might pay you to ask their opinion before doing anything to it.

They do respond to hard pruning. Where I used to work there was a whole bed of them cut back every year to abut two feet. I would just trim off the sides and let the top go up.