The report is done and one thing is still theirs. A mark they asked you to find is not where they thought. A neighbour's fence does not match the line you measured. Your assistant drafted a careful summary of the whole survey. They already have the report. What they do not have is a question they can answer from a kitchen table.
This is not advice about title, boundaries as a legal matter, or what they should do in a dispute. It is you, the person who was on the land, asking for the next step only they can authorise.
One finding, as a cold reader would hear it
Write for a cold reader. The address, the date you were there, the finding in the words you would use on site — where you stood, what you measured or saw, how that differs from what they asked you to look for. Then the one question: authorise a return visit to mark X, confirm that the post they mean is the south one, or hold until they have spoken to someone else.
Do not ask them to interpret the survey. Do not ask them who is right. Those are different professionals, and they can take your description with them.
Do not attach the report. RelayLink does not carry attachments. Describe the finding. The drawing stays where you already deliver drawings.
A briefing is the right shape because the ask sits in a field and the finding is not braided into a story.
Assumptions that turn into a second visit for free
State the assumption. You assumed the post they photographed is the one they care about. You assumed they still want the line marked, not only described. You assumed they can be on site on the morning you named. A labelled line gets a no. A paragraph about "the usual case" does not.
If your assistant inferred a dispute from the tone of their first email, mark it inferred. Tone is not a brief, and it is not something you should write into a package they will forward.
You approve it; they reply from email
You review the wording. Use the names they use for the lot. A nickname you keep for the owner stays on your side.
They do not need an account. They can reply from the mail. If they already have an activated account, a new thread needs an accepted pair. If two people own the land, one briefing still goes to one mailbox. Say the other person should see the finding.
After they answer
Write back the next step in the same words, and ask them to correct that sentence. Then do that step, or wait. Do not add a second mark "while we are out" without a second ask.
The report can still be long. This email is the one thing the report cannot decide for them.