Hard to say without seeing it - the way to tell whether its sucker growth or not (its not always easy to tell) is to find the point of origin - if its a sucker it'll be coming from below the graft, the lumpy bit at the bottom. If its coming from above, then its just a new shoot that may eventually look just like the rest of the plant.

Thanks for your reply Bamboo - it does seem to be growing from below the graft, but I thought a sucker had seven leaves and no flowers(buds)? I have vaguely heard of 'sports' appearing but am ignorant about them.

Some have extra pairs of leaves, some don't, sometimes the growth is a different shade of green, sometimes it just doesn't seem quite right, but the acid test is where the shoot is coming from, so if its below the graft, it's not one you want because its coming off the rootstock. If you're still not happy, you can wait till that stem flowers - should be dog type roses instead of the bush you bought, but don't let it, or any others, take over, because it will take energy from the grafted rose you paid for.