What do you mean by "some time"?
If its more than a few months, I'd dump the fuel and fill with fresh and a quick dose of Halford's best injector cleaner.
If the injectors are blocked, why would it only run rough at low revs?
Also, have you checked the battery? Sounds counter-intuitive I know, but my Sprint would never run at low revs and I discovered that the battery was f****ed and a new one cured it completely!