While orienteering the other day I spent ages looking for a control then finally found it in the wrong spot. Should I have moved to the correct spot so everyone who came after me didn't have to suffer like I did?
I'm assuming from your letter that you didn't actually move it ? If this is the case, you did the right thing. The placement of the controls is the organisers & controllers responsibility and even if you are completely right in your assessment of the placement (and remember, you may not be), moving it is not the correct course of action.
If you happen to be one of the first runners to encounter the problem, and/or you are close to the arena, you could consider returning to the start to notify the organiser who can then decide what action to take – but this is totally up to you (and remember, you might be wrong!). Either way, leave the control where it is.