That's brilliant, thanks.
"I don't know if this partition type changing will help, but..."
Neither do I and I'll be trying the bootable Partition Magic first, but I like to have a couple of options and there were a couple of replies on the messageboard about how this had worked for the posters.
My 3rd option seems to be:
Install windows over the top of linux on the new partition, then install PM on that and use it to switch the active partitions.... (then repartion and reinstall linux properly...)
A bit long-winded but it should work if all else fails.
So long as I dont manage to blat the windows partitions with fdisk that is...