As the others have said, the "leginess" is mostly likely due to lack of direct sunshine. However, since it's a dwarf variety that means it's a hybrid. As you prepare to transplant it, inspect to see if any shoots have come from beneath the graft knob, which will be noticeable just above the root area on the main stem. If one or more canes are growing from below the graft then you may as well trash the little plant. The plant has "reverted" and is growing from the wild rose root. The canes will never produce blooms and will take up the rose's energy in producing these fast growing canes.
Let's hope that's not the problem. Additionally, here in the U.S. there are smallish rose fertilizer "spikes" one can buy that are inserted into the soil around the plant (maybe two or three). They're formulated just for roses and will last the entire growing season.