Rather more perforated than first feared and (as feared) the balance section is nowt but rusty metal. Neighbour (skilled welder( is patching it. We're planning to just remove the balance pipe altogether and patch over the hole on each side collector. I'm assuming that the balance pipe just reduces noise by a minor decibel or two and boosts horsepower by the same infinitesimal amount. If all else fails I'll stick a four-into-one on it for Chimay. There's one on Ebay but the robbing cnut wants £25 for postage (parcel post is under a tenner).