there should be no cost to short positions, if anything you should be getting paid interest on the margin when interest rates are a great deal higher.
mark to market explained below as is on cmc website.
Your Daily Statement
Mark to Market
Trading profit and loss is realised on your account balance on a daily basis. We will mark your position to the CMC Markets price at the end of each day and calculate the profit and loss for each of the positions you hold. This process is known as 'Mark-to-market'
We will then re-open your position the following day at the previous day's close price (less a small financing adjustment).
View this example
The above example shows a "Bet Position Summary" taken from a daily statement. In the example, there have been two trades on the account that day, Buy 2 UK100 @ 5236.00 and buy 1 UK100 @ 5209.00
The latest/close price we take for our rolling cash instruments is at 10pm London time. Daily trading P/L is calculated by working out the difference between each trade price and the latest/close price and multiplying that difference by the relevant stake for each trade.
The Buy 2 @ 5236 is marked against the close price of 5191. This is a difference of 45 points. With a stake of £2 per point, the P/L on this trade is 45 points x £2 stake = £90. Because the close price was lower than the trade price and the trade was a long trade or 'up-bet', the £90 is a loss.
The same is true for the second trade of £1 per point, where a difference of 18 points has led to a loss of £18 on the account.
The statement summarises the total P/L for each instrument and then totals the P/L for all instruments. In this example, the P/L for all instruments is a loss of £108
This figure of £108 is seen again in the cash activity section of the statement with an entry header of "bet mark to market". In the example below, £660 is the cash carried forward figure from the previous day's statement.