I have taken advice from several sources,including the one below.Many thanks to those who have messaged me but for their own reasons didn't wish to post on the thread.
Palpable errors are a tricky subject for the firms. Basic contractual law does not allow for such a situation as the one that you outline. In simple terms a deal is a deal. The various firms all say that they have the right to cancel trades for whatever reasons but legally they are on thin ice as contract law and, more importantly, the Financial Services and Market Act of 2000 appears to state otherwise. The Act appears to state that all spread bets are enforcable by either party. So, in the case that you outline, you would appear, by law, to be able to enforce the contract which they entered into with you (be that the opening or closing of a bet as technically they are the same thing ie 'a contract').
I'm guessing that Capital phoned you and told you what action they are taking ie removing funds to the value of?
Technically I doubt that they can really do this - all they can do is phone you and ask that you might act reasonably and in good faith towards them - in other words to 'adjust' the closing level to one which is more in keeping the market conditions at the time. Ultimately the choice in this matter is yours and not theirs since it is them that is seeking to break a contract (the closing order) which exists between them and yourselves. The problem from Capital's point of view is that the individual spread bet executions are individual contracts in their own right regardless of the main contract which is made up by the Terms And Conditions of The Customer Agreement. Because the law recognises spreadbetting contracts the firm would find it hard to show that they could use a wording in another document (ie the T&C) to cancel a contract (the spreadbet contract note) which is a seperate contract in its own right.