@Booz:
This is the site which used to contain the interview I mentioned above:
Financial Spread Betting for a Living
I haven't checked, but it might be the one labelled "Interview with the managing director of Capital Spreads".
CapitalSpreads Interview
If it's not the one I'm thinking of, I'm pretty sure it will be of interest, anyway. ISTR there was more than one interview with him, or maybe others in the CS stable.
There used to be lots of other interesting stuff on there, which may still be there if you drill down a bit. I've just not checked recently. I used to find that a pretty interesting site. I don't know if anyone keeps it up to date these days.
I know that CS is not Tradefair, but TF is a "White Label" of LCG (who own CS), and I believe uses the same "trading engine" under the covers of a different "wrapper", so in essence, I think it follows the same rules. The main difference is the "look and feel". TF used to have a nice, simple look and feel, which was why I liked it. Not tried it recently. CS was also ok, but at the time, I preferred TF. It also seemed a bit faster, for some reason.
If you avoid overtrading (in size and frequency) and avoid periods when the market is going crazy, or if when you know some bombshell is likely to hit the market, hopefully you should avoid major disaster.
Or you could just go onto the CS thread and ask him yourself (as it applies to CS, naturally
).
EDIT: This is not a complete, answer, but goes some of the way:
This is from the Q&A with Simon (and Angus) of CS (n.b. not Tradefair):
http://www.financial-spread-betting.com/Spreadbetting-industry-faqs.html
(Page 2)
Q.: All clients must have the available resources on their account to cover the risk up to the stop loss plus 20%...why the extra 20%?
A: The plus 20% is to take into account occasions when the market gaps on opening. If a market closes the day just a few pips away from your stop but then opens the next day 20 or 30 points through your stop then Capital Spreads requires that there is a reasonable possibility that you have enough money to cover the slippage. If we only took the exact amount of money for your stop then we would end up with a lot of people owing us small sums of money.
And somewhere I think he says that they rarely end up with people owing them money.
EDIT2: You might find some of this section interesting also:
http://www.financial-spread-betting.com/reviews/