Thanks for the replies.
Using the replies, googling away and following many blind paths eg spurious
message on mkswap 'no uuid'.

The final solution was
a. remove the fedora swapfile from fstab in Ubuntu-> i.e forcing Ubuntu
to use the newly create SWAPFILE only.
b. edited menu.lst and added the following to the initrd entry for fedora
(this being the original swap file for fedora install)
c. reboot into fedora,
d adjust the fedora fstab to make sure that the swap line was
'LABEL=SWAP-sda13....swap swap ....0...0. etc'
e. swapoff
f. mkswap -L SWAP-sda13
(ignore no uuid message / warning info)
g. swapon -a
h. reboot.


I then rebooted in Ubuntu and did a swapon -s to check that all is still OK
then rebooted into the updated FC7, checked there, and reboot into original
installed fedora and checked that it is also OK

Also checked to ensure windows XP prof still worked OK