Hi Dave,
If you can transport your boards you might want to see if a local sawmill can at least mill then to a more manageable size for the mere mortal woodworker?
Even resawn to ~ 20mm or 30mm would make them far more marketable and given their age, there should hopefully be little or no movement.
Another option would be to look up some chair makers (such as James Mursell) as elm is a traditional wood for windsor chair seats.
Auctioneers are a further option and H J Pugh have sold large chunks of elm in the past so you could always check their auction results to see if the prices match you expectations. You probably need to have a look at the auction before last when I think there were several large pieces of elm.
Hope this helps.
Glynne