Marjorie's seedling plum.

[Vimto]

I have a ten year-old Marjorie's seedling plum tree in my garden which fruits reliably every year and produces around 20-30lb of delicious plums although this year yield was down but quality was excellent. Today, I noticed a young one growing a little away but just underneath the tree. It is about 2 feet high and its leaves and bark are identical . I think it has grown from seed but it's possible that it is a sport which has grown from the roots but I doubt this. I have no idea what the rootstock is on the parent tree (yes, I know, I should keep records) but it was acquired from a specialist grower.
Is this a viable tree that will grow true and produce plums in due course or should I destroy it? I can easily move it to its own spot but in view of the room and time I will have to give it, will it be worthwhile or am I wasting my time? Anyone with experience in this field please advise.

[johnk]

It's unlikely to be any use as a fruit tree. Seeds rarely come true and suckers are from the roots and will be the same as the tree it started as.

[Vimto]

Thank you John; I thought maybe because it's called "seedling" it may grow true in the same way as "pippin" apples.

[Chipchopper]

I'm inclined to agree with johnk, and think it's most likely to be from a root sucker which your plum variety was grafted on to, back in the nursery