Thanks, guys.
I have a few demijohns (1 gallon). The reason that I did not want to use these was because I only have the one demijohn heating pad and I would effectively have to have two heating pads to accomodate the two demijohns.
I do have a brewing heating belt, but it's for wrapping round either a carboy or a large fermentation bin (bucket) and would be too large for a demijohn.
I'll have to see if I can wrap the brewing belt round a demijohn somehow and hold it it place with a clothes peg perhaps, then I'll have the heat pad available for one demijohn and use the brewing heating belt for the other demijohn.
I used a 23 litre bucket to do the first stage of fermentation because I had nothing smaller. A bit late in the day to ask, but would this be detrimental to the small amount of wine I'm making, because of the considerable amount of head space?