Hi Ian,
I use 300-21 Michelin tubes. They come in several grades, but I use either the Heavy Duty or the Super Heavy Duty style - not sure what they are called. The super HD one comes in a huge box and is very subtantial indeed.
They are expensive ($30-35 last time I bought one), but given the consequences of a flat (or worse still a blow out) on a Beaded Edge tyre (it comes off the rim) I make the effort to put in the best available tube.
By the way, often BE tyres come with tubes. I NEVER use these, other than to wheel unrestored bikes around the shed. I always buy a new Michelin tube. Of course other major brands also produce good quality tubes. Usual disclaimers apply.
Cheers
Leon