Thanks Richard but there's no problem with the transmitter. It's a Futaba 6EXA which I use for three different models including St Columba, I have no problems with any other, just the pusher tug.
In desperation I'm going to swap ESCs from St C and put them in the tug. Then if things don't improve I'll swap the 'genuine' futaba receiver from St C as well.
If it still fails there's always the lump hammer...:-)
Scottie