Thanks for your helpful replies, Gaius and others.
I'm a beginner not only in SU but in Roman buildings too and have no training or special knowledge, I live just outside London. I'm afraid I don't know what the Batch Museum is, did you mean Bath Museum?
I tried to follow how you modelled the arch but I can't see any way to run the plug-in. There is nothing on the menu and I can't see any instructions either, yet you all seem to be able to do it. Can you point me at some instructions on how to run plug-ins, please.
I have the free version if that makes any difference)
Meanwhile I looked at the other threads. The Tiled roof is great, thanks and will really help. I'll probably go and photograph some bricks and mortar and have a go at my own textures as well, I knew that pile of old rubble in my garden would come in useful one day. 
Your Basilica is very impressive. I wasn't going to attempt any monumental buildings and the ones I'm currently interested are mostly single-story utility buildings such as storehouses, barracks, farm buildings, granaries etc suited to a Romano-British settlement or frontier town. I think only some buildings would have arches, and many would have rectangular openings. I think most walls would be stuccoed, even stone ones and some would be timber-framed, wattle and daub and some roofs might be thatched.
This reconstruction is about as monumental as I wanted to go, but I suspect it has some modern techniques in it. You might find it interesting for its own sake
http://www.gallica.co.uk/villa/photos_high.htm
If I come up with anything worthy, I'll post it.