Two things, get a solicitor to check the legal status of the right of
way. Get a drainage engineer to checkout that tank for you - unless you
know how to yourself. I'd consider that at least as important as a
survey. I had an extremely expensive fix-up on a cottage I bought off a
little old lady that assured me all was well ("Only needs desludging
every 6 months", failed & backed up in 3 weeks, pump-out guy says "Oh
yes, it's wrecked, she had me round every 2 weeks").