@stuartk , There is no reason you should not be able to make your current situation work. Your seating will be further back in the hall, images will be smaller but that is all. 1st off the image should never extend beyond the speakers unless there are phasing tricks being used like in Roger Water's Amused to Death. The effect of imaging beyond the speakers is due to poor control of early reflections off the side walls. Some sound absorption is in order. Point the speakers right at your head. The next problem that affects proper imaging is imbalance between the channels. Your brain locates noises by volume and phase. If the 2 channels do not have the exact same frequency response curves the image is smeared, out of focus as some frequencies are louder in one channel than the other. No two identical loudspeakers have exactly the same response curve. Then you really screw things up by putting them in different positions. Now each speaker sees a different acoustic picture. I demonstrated this to a friend just yesterday. He had heard my system and wondered why his could not create a similar image. I measure each channel and showed him the curves and in the midrange they were wildly different up to 10 dB apart in places! The only way to contend with this is digital EQ where you can adjust each channel independently to match within a dB from 100 Hz to 10 kHz. He is going to get a MiniDSP SHD digital preamp which can contend with this problem at a level he can afford, appropriate to his system.
The speaker and room are parts of the same organ. There are some rooms you are never going to get a decent image in. Both speakers need to see identical but mirror image environments as a starting point. The more variation there is between speaker positions the more impossible things become. Windows in particular are a major problem. They resonate like drum heads at frequencies that vary depending on the size of the window and stiffness of the pane. I spent three grand having a window removed. What a breath of fresh air that was.
In short, the best imaging requires symmetrical speaker environments with close tolerance loudspeakers, proper acoustic management and absorption at early/first reflection points along with digital signal processing to touch things up. Anything else is wishful thinking or shear luck.