Other costs, in addition to the spread, that you need to consider when spread betting are the daily financing charges. Financing charges represent the cost of borrowing the capital that is needed in order to open a position. Positions that remain open over-night are subject to those charges, incurred on daily basis.
A long position attracts the financing charges, as effectively, by depositing initial margin, you borrow capital from us to cover the rest of it (i.e. the difference between the initial margin and the total value). For short positions on the other hand, financing interest may be credited to your account, as the opposite applies, meaning, you are lending capital to us.
It is worth noting that those charges do not apply for any instruments with an expiry date (such as for example Commodity Futures) as the price in the underlying takes that already into account.