How did Harris and Jerome get blackmailed and describe their reaction?

The man who attempted to blackmail Jerome and Harris didn't make himself clear...... thus, Jerome and Harris were ignorant of what he really wanted..... money.

We stopped under the willows by Kempton Park, and lunched. It is a pretty little spot there: a pleasant grass plateau, running along by the water’s edge, and overhung by willows. We had just commenced the third course—the bread and jam—when a gentleman in shirt-sleeves and a short pipe came along, and wanted to know if we knew that we were trespassing. We said we hadn’t given the matter sufficient consideration as yet to enable us to arrive at a definite conclusion on that point, but that, if he assured us on his word as a gentleman that we were trespassing, we would, without further hesitation, believe it.

Of course, we never saw him any more, and, of course, all he really wanted was a shilling. There are a certain number of riverside roughs who make quite an income, during the summer, by slouching about the banks and blackmailing weak-minded noodles in this way. They represent themselves as sent by the proprietor. The proper course to pursue is to offer your name and address, and leave the owner, if he really has anything to do with the matter, to summon you, and prove what damage you have done to his land by sitting down on a bit of it. But the majority of people are so intensely lazy and timid, that they prefer to encourage the imposition by giving in to it rather than put an end to it by the exertion of a little firmness.