It is obvious where the oil has come from - from the garage at the back of the house (yes, the same garage whose presence and "increased fire risk" made it almost impossible for us to get house insurance). The manhole covering our connection to the sewer is at the back of our property in a piece of land where they dump rubbish - in bins - but the manhole cover was broken so some of it leaked through. Apparently everything was "compacted" to such an extent that it must have taken years to build up to the point where it was causing us problems!
So all the yuck has been pumped out, the drain has been flushed out, and we just need to buy a new manhole cover. To further please us, the plumbers went round to the garage and spoke to them about the problem, thus saving us the need to do so! I'm just... really impressed. If you live in south London and need a plumber, call Maintracts Services - they rock!
Tell you what, though, I've entirely changed my mind about retraining as a plumber so I can charge people £110 +VAT per hour instead of the measly £25 an hour I charge for science tuition. Just the contents of our down pipe was enough to put me off - and that was our down pipe! I think the extra £85 per hour is perfectly justified as a tax for dealing with excrement. Hell, even the contents of the U-bend in the kitchen are disgusting enough...