Switching to local unfortunately also attenuates the WM signal too sadly!
@tri-comp - The previous owner re-capped it apparently, the SW1 works fine on SW, LW and MW with no motorboating or other problems. Thanks for the offer of the service manual by the way!
@steve - I have a feeling the SW1 has a problem with strong signals - but with the state the radio was in I wanted to rule out any other problems. Didn't want to shell out for a new aerial (if I could find one!) to find it didn't cure or help the problem
It does seem odd though that a small, (then) premium-priced radio would suffer from this kind of problem on FM though I guess it's primary purpose was listening to the non-FM wavelengths. Still very fond of the battered little Sony though!