Thanks Tom.
Roofs 1 & 2 are fairly pitched roofs that I can't connect with the 'connect/combine' tool. I have now use the 'clip surface' tool but you will still see that the cursor doesn't highlight the object when I do try the 'connect/combine' tool.
I was also hoping to combine the flat roofs 4 & 5 to the pitched roof 3. I haven't tried your proposed method using Slabs + the Subtract 3D Object... command, but will give that a go.
Is there a maximum size of file I can download? At present it's 67 MB. Having problems downloading it and I've already reduced the size quite a lot.