Well it seems like the issue whereby my disk group was showing free space on 1 disk in the diskgroup but out of space on the other disk has turned out to be a bug. The details of the problem are available in the last post.
Oracle support are now saying that we are being hit by Bug 4380450, which is to do with:
Unbalanced space usage if diskgroup has only two disks both of different sizes.
This bug is known to affect 10.2.0.3 (the version I was running). and is marked as fixed in 10.2.0.4 and 220.127.116.11. The disk group must have exactly 2 disks and the disks must be of different sizes, then it is possible you will have unbalanced data allocation, which in turn can lead you to filling one of the drives, which means you can no longer allocate disk space from this disk group. If the data was balanced you would still have storage capacity available.
The workarounds on this bug are to perform a manual rebalance or use disks that are the same size – advice which I would thoroughly agree with!