I think I may have found the reason for the out of memory errors myself!
The software seems to allow you to have 4.7 Gb for the project. I have just tried again, but limited the project size to under 4.37 Gb (i.e. 4474.88 Mb).
This time at least it burned a disc, but I am still not getting any sound on most of the clips - the finished DVD had sound on the first and last clips only. All the clips used have sound ok when played on the computer.
Any ideas anyone?