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Thanks for your question. You will have to check the titles for the properties. There may be a wayleave for the soakaway in which case the neighbour has a right to have it on the land. Or a wayleave could have been established by the passage of time if its been there for a long time. If that is correct and that is the tenor the legal advice he has had then it is likely to be correct. I hope that helps. Please leave a positive rating so that JustAnswer credits me for helping you today.
Current regulations aren’t relevant if it was OK at the time. And if there is a right by virtue of the title or passage of time, no there is no trespass. See my answer above.