Having welded up mine a week ago, I can concur with this. Sill rot on these things is likely to be the tip of the iceberg, a small hole generally inplies that the whole area around it will be totally rotten and a wider area still, will be "manky" and not weldable. If it's bad on one side, it's normally only slightly better on the other. If it's in the early stages, you should catch it quickly and try to nip it in the bud. P.S. have a look under the carpet thingys in the rear, inner wheel arches, they tend to rust out above the flange and where the brake pipe holder is attached.