You need to ensure that the legal conveyancer (or your solicitor if you wish to spend more money than necessary on the transaction) who deals with your purchase ensures he establishes that you have a covenant over the farmer's land to drain into the existing septic tank.
There are no retrospective Building Regs that have been applied to existing septic tanks already in place - only that if the local authority considers one to be a public health hazard, it might put a prohibition order on using it. You really need to have the installation surveyed as part of your full survey - or DIY of you know what you are looking for.
There is no way you can stop him erecting a shed to spoil your views by relying on the local authority Planning Process. Forget a shed - what about a full-blown farm buildings? - these things can be like hangars. Farmers have incredibly broad rights that enable them to apply for and get PP for new large buildings.
If you are serious about this as a constraint, your solicitor needs to establish a legally-binding agreement with the vendor that he will not build a structure on a area that should be attached as a plan - this needs to be part of the contract of sale that you both sign. That is your best legal protection against this sort of thing happening.
PS Don't forget about trees / high hedges - he might get spiteful in the future in put a line of Leylandii in - best cover that angle on the agreement too.