Shit. Literally.
Well actually I failed to fettle it, so a) the septic tank emptier truck appears and does its sucky shitty stuff, but still no drain flow, despite enthusiastic rodding on my part, then he suggests getting the jetting team in, which I agree to.
So b)Jetting guys appear in van, and give me a lesson in rodding techniques for this precise situation, and promptly clears it, which a resounding suck and splat. At least now I know the technique, should my less than optimal drainage layout give further trouble. The suspicion is that my "drainage field" i.e. the bit of John the farmer's field behind, where my septic tank discharges into, may be gunged up permanently. this would require getting John to agree for me to spend thousands on digging a series of trenches, applying many tonnes of graded aggregate, perforated pipes, geotextile, topsoil, and re-seed the damaged bits of John's grazing field. This will have to be done during the period when he has no stock in it, to allow the grass to recover from being grazed.
Oh, the joys of country living... Thats £240 down the pan today, and several thousand some time later this year.