"When you already hold an open position in VOD (say) it is more probable that your next trade will be to close the position. That is why ask for a quote of the spread before I give my account details"
I hadn't thought of that specifically, but makes good sense. I'd rather deal with an exchange traded market than trade with a company that has a book to balance.
Remember RobinT saying at the LIFFE meeting that he traded with a SpreadBet company and hedged in the market. Net result was that profits (SB generated) were tax free, hedging losses (Market Generated) were tax deductable. Tax man swallowed it, apparently.