Twenty years or so. What difference does that make, or is this where you play Barry Scheck? ;)
How many labs confirmed this relation?
At this point, probably none. Confirmation will no doubt be forthcoming, as other researchers attempt to replicate these results.
What is the basis of calling one species "related" to another?
Genomic similarity.
Is that a subjective question?
No, it's a matter of statistical tests.
How often are mistakes made?
Not very often, but they do occur, which is why replication is important.
It would be interesting to know how extensively British flora was chronicled in the year 1703.
These fine folks can probably help you out.
If the variety of recorded plant inventory was substantially lower than it is today, how are we to be sure whether an introduction or a discovery occured? IF we are counting on a ship's manifest as the basis for an introduction, how accurate are the drawings of the plant in question? Would those standards of accuracy be allowable today?
We certainly don't know any of that to be the case, so it's hardly worth speculating about. However, we do have expert testimony about the ages of these species available to us, if you'll cast your eyes upward momentarily...
Against a backdrop of few new species being found, the discovery of a "new" species can be attributed to the nearly complete, but still incomplete status of the record, or to the introduction of plants from abroad, or to genetic fraud without having to conclude evolution has occured.
IOW, every option is open but the obvious. And if all those possibilities are ruled out, you will then - quite rightly, too, I must say - point out that we cannot dismiss the possibility that God Himself personally placed this new plant right there, or that this new species arrived via meteorite from Mars, or that some fiendishly clever genetic engineer whipped it up in his lab and planted it right there next to the river, and so on and so forth.
But perhaps we ought to limit ourselves to the most likely explanations...