It's not 30 years!!!
1901 the stones were straightened up and set in concrete.
During the 1920's there was renovations which included moving 6 of the stones and re-erecting others,
1958 cranes were brought in to reposition some fallen cross stones,
and in 1964 4 more stones were re-positioned.
Here's an an ariel view from 1906 and present day from the BBC
English heratige appears to have missed the above out of the guide book for some reason?