I've got the inlet manifold off my Cobra at the moment to replace a leaking gasket and I'm planning on refilling with Evans.
You don't need to use their flush fluid though. Just drain the water from the system and half fill with undiluted glycol based
antifreeze and bring the engine up to temperature. Drain it off and then fill with Evans. This advice came from Evans themselves, so I have no doubt about its validity.
One of the benefits of using Evans, especially on an FE, is that it runs in an unpressurised system, so the notorious inlet manifold joint isn't subjected to high pressures, at least not from the cooling system.
Paul