I would say contact the Council - they empty them (or contract it out) so they will probably have someone who has knowledge of them. When we bought our croft in 1994 our old fashioned concrete box type septic tank was blocked; fortunately our local plumber knew about them and went down into it (yuk!) and discovered that the 'T-piece', whatever that is, was broken. He replaced that and it worked perfectly till the day I moved and still does to my knowledge.