Sandy lane is the problem end of Redruth. It's also very old.
1. The Ordnance Survey maps only give you a part of the picture as many of the earlier shafts were obliterated.
2. The mine plans of Wheal Sparnon and Pedn-an-drea cannot be considered to be complete. (and are not complete) although one of them (I forget the code) is very good - it's a bit more of an overview.
3. The area maps MRO-A contain some of the tin bounds areas. These don't contain shafts, but they were recorded as workings. Anything could occur within them.
4. A good indicator as to the whereabouts of the major structures in the area are the Symons "area maps". These show the main lodes or where you're likely to have a downstairs.
All of these combined are good enough to distinguish between slight risk and moderate/severe.
The areas you speak of do have issues and it's a fair probability that you would need to factor in a part of a drilling investigation in order to clear a house for lending (They insist). This is in the order of £3k. Your mining search will detail all of this. If you are a cash buyer, you would be wise to go down the route that a lender would (making sure things are watertight) because if something goes wrong, you will find out about all the small print in your insurance/searches, etc.
I wouldn't attempt to do this yourself, as whilst you may get lucky, your conclusion will not be backed up by PI insurance when the hole under your house opens up and then proceeds to affect next door as well. It's a legal minefield (geddit) and things are NOT clear cut.
Sandy lane is an area where you need to consider a large amount of material in order to "clear" a place (for lending). A lot of the field boundaries have changed (since the bounds map) and even the "interesting plan" has changed hugely. Streets gone and redeveloped. It's quite a challenge to say "this is definitely here". Sticking it all together with GIS software is possible and makes things easier BUT some of the features have been sketched on the map approximately. If you use these as your "control points" you can get some very strange things happening.
I'm always happy to talk/explain the detail of an area to people BUT this does not come with insurance backup.
There are plenty of people in Cornwall who find it acceptable to "get enough information to do it themselves". It's rather like giving an architect's preliminary drawings to your builder to cut out the expense. It is really poor form.
When you've communicated with whichever mining consultancy you are dealing with, they all (well, nearly all) contain someone who will answer your queries in the sort of level that any curious purchaser/vendor would require.
Lots of houses have been drilled up that end. Lots of shallow workings have been found. Lots of unrecorded shallow adit levels exist. Keep your eyes open and notice the collapsed wall by St Day road roundabout and the wobbly wall further back towards Mount Ambrose. When you look at a property, note any 10cm patches in the concrete around the property. Note any piles of stuff that (in that area) looks like road salt. Also, do not do anything in that area without seeing, or getting ALL the required paperwork with your own eyes and if there are any questions, ask a firm. They will either say "You need X information and we can do it" or "What you have is sufficient".
Hope this helps.