You can be 99.5% certain they will be "pure" R. eglanteria. With many other roses in your garden there is a possibility some seedlings might be hybrids, but I bet they will be rare, if any.

R. eglanteria does not have the normal splitting of the chromosomes where both parents contribute equal numbers. R. eglanteria has 35 chromosomes. The egg gets 28, the pollen gets only 7. The signifance in your case is that even if a foreign pollen did the fertilizing to form your seed, the resultant plant will still look very similar to the mother.

Paul and Henry,

Thanks for getting back to me on this. I was a bit confused because I have some R. eglanteria hybrids (Lord/Lady Penzance, etc). I guess I should investigate these crosses.

Robert