Of course I didn't think about going back to previous scenes!

I've used exterior materials brick, stone, wood, and yes some 3d trees. I will try purging components but have been quite careful to delete what's not needed. Its almost finished now as I don't think it can cope with much more. Thanks again I found your suggestions useful.

Dave Thank you for your helpful response. What you suggest sounds like it makes sense, i'm going to try this on a practice model. My model has lots of groups and components and is now running very slow despite single faces and lowest polygon counts therefore didn't think it would paste too well into another file. I think I understand the process now but can you tell me, is there anyway to escape out of the process putting the model back to its starting viewpoint without closing the file and not saving?

Maybe I'm doing this the wrong way round but I'm trying to match a jpeg using the 'New Match Photo' onto the front of my model and then adjust so that the perspective is right. However the image comes in in the background and much larger than the model which is in front. The red green vanishing points seem to move the model not the image. How do I resize and position/project the image onto the model (couldn't find push pins). Also how do I escape back to original position when I've totally mucked up the perspective. I've tried deleting the scene but doesn't delete the skewed perspective which means closing the file and starting again.