It's quite interesting just how little septic tank you need in practice.
We also used to live in an old keeper's cottage. Likewise, the septic tank received water from the roof plus the single bathroom and kitchen sink.
After five years of living there, the toilet stopped draining, and we found that the pipe was backed up with sewage. We suspected a tree root had grown through the pipe, but actually it turned out to be a disposable nappy about 20m downstream of the toilet!
The nappy can't possibly have been ours, and must have been there for at least 5 years. Although it let water seep through, it completely blocked the pipe with respect to solids. This effectively meant that the pipe had *become* our septic tank. However, it was five years before we actually had any problems!!