Long time since I worked on Rover V8's but sounds to me like the float or the needle shutoff valve might be the culprit. There could be a little bit of dirt preventing it from closing properly (even if you have fuel filters), the valve could just be worn or maybe there is a leak in the float itself. Generally I do not favour Strombergs and their troublesome diaphragms so you might want to consider swapping them for SU's at some point.
