[Arran]

Which set has the motorboating issue? The 6S229 or the 6S203? One thing to check in sets like this that have tubes with grid caps is the grid cap leads. The clip they use to connect the grid lead to the cap often develops a bad join between the lead and the clip either through the solder breaking down or the wire breaking inside. There are other things that can cause motorboating, the main ones being an open ground on a bypass capacitor or an open output filter capacitor. Sometimes the IF alignment being off can cause squealing or motorboating as well, another thing could be the primary and secondary leads of an IF transformer being too close to each other.
Regards
Arran