Ian – A few random things to check, if you haven’t already: Reduce RF gain or select the MW notch, just in case something is overloading the front end. Overloading often causes the noise floor to shoot up. Check the braid of the coax is connected. If it has come off at the receiver end, you will get noise from your own computer etc. Check you haven’t switched on something nasty! We have two LED room lights, one in the kitchen, one in the bathroom and although they don’t have a dramatic effect across the bands, they do radiate narrow-ish bands of ‘FM hum’ at harmonics of the switching frequency.
I can’t think of anything else right now. Good luck with it, it’s a growing source of frustration.