Ex dividend means they are valued without the right to receive the next cash dividend that's going to be paid.

Eg, say the dividend is 3p a share. Thursday they are valued at 105p per share, cu-div, ie including all the value of investments in the fund and cash in the fund's bank account less the liability to pay out a load of cash to all the shareholders as a dividend that's already been declared. If you buy at that time you are paying to buy all those assets and liabilities and you will be receiving that 3p cash dividend. But it costs you 105p to buy in.

And then Friday, they are only valued at 102p per share, ex-div (without the right to the dividend), because they have locked down which specific investors are going to get the 3p cash. Clearly, the shares are worth 3p less if when you are buying or selling the share they no longer carry the right to get the 3p cash.

By all means, buy in on Wednesday or Thursday and pay 105p, and sell on Friday or the following Monday for 102p, later receiving the 3p dividend in cash. But, that's just getting back your 105p, for a net overall break-even. The capital loss is equal to the taxable dividend income received. If you don't have enough spare dividends allowance, you might need to pay tax on the income. So, you are either break even (there's not going to be any free money magicked from nowhere), or net loser, (because of the tax). It's not a double dip double dividend win.

Aha a flaw in my plan!!

Thanks for your help B