I recently had my diesel Range Rover sport serviced but the garage didn’t re-seat the fuel filter correctly causing a leak of fuel. At the same time, my engine light came on and it turned out to be clogged/faulty EGR valve. My question is, could the fact I drove several hundreds of miles with an unsecured, leaking fuel filter have caused the EGR failure?
No, they are not connected in any way.
The EGR deals with exhaust gasses. They can, over time, get plugged with soot or possibly have a mechanical failure (depending on design). The EGR only comes into play after power generation, and has no connection to the raw fuel or fuel delivery system. Unless the leak was directly spraying on the EGR, there's no way a leaking fuel filter could have caused this issue.