After reading the recent posts on the travelcard I faxed London Transport to see if I could purchase a 7 day Zone 1 travelcard on arrival at Heathrow.
I have just received an email reply and no multi-day travelcards are available to visitors apparently - unless these are purchased online with exhorbitant overseas delivery fees. Also the Zone 1 travelcard is not available this way - only the Zone 1-6 travel card which is double the cost.
Anyway, we leave soon and there is no time to order one.
London transport should update its website. I had checked it out carefully (I thought) and there was no indication that these restrictions applied.