Hi Joe,
I used to manage a fantastic little cherry/english elm woodland. The English elm were beautiful with fantastic long, bolt upright straight trunks. They have a much straighter form than the slightly irregular growth of Wych Elms. I haven;t been to the site for a couple of years but last time I went everything seemed in good health.
Wych and English Elm naturally reproduces from suckering and can be regenerated through pollarding and coppicing somewhat. Have a careful hunt for more elm around in your woods because its fairly likely you will find some close by growing off the same rootstock. Another point to consider is that Dutch Elm Disease only starts to kill trees when they reach about 15 years old. Thats why you tend to see dead hedgerow elms of a certain size. Bearing this in mind there may be some form of resistance by coppicing before they reach this age and manage on a 10year rotation.
Unless the planted tree was infected or had beetles on it being imported into a woodland from an infected site (which is unlikely seeing as beetles couldn't survive on small trees without bark) I personally wouldn't see planting a couple more being a problem (in my opinion). Although, you may want to consider focusing your efforts on planting another type of tree completely as dealing with any elm is a risky business.
All the best
Pat
Treeation - Tree Surgery & Woodland Management
www.treeation.co.uk