Ive just recently repaired the end sections of my A frame x member (body off). Also found the top of the chassis rails had rotted through underneath ! After I completed the repairs I capped the rear strengthening web the the same as Ian's pics. Another defender design flaw.
Also as it was impractical to take the chassis back to bare metal and paint, I gave it a thorough scrape and wire brush and brushed on waxoyl underseal. You can also buy this in aerosol for the difficult to reach areas.
I know you can buy this crossmember but I forget who makes it, I remember it being expensive so I repaired mine.