Normally 63mm pumping discharge
guide rails are good as pump slides down rail and connects easily to tank good if ever go wrong for maintenance.
twin pump are also good systems
have you tried marsh industries at Kettering way they are also stocked at sde Milton Keynes
Can you not use a pumping station to pump yours to a manhole at your in-laws at say 450mm deep and have natural fall from there to the main sewer in the road
Generally speaking foundations are 1m deep with a min off 450mm of concrete Did you have a structural engineer design your foundations? And did you set them a datum level to work too?