Hi,
In brief, the 50:50 rule applys to jointly held property (for this purpose property means shares, bank/building society accounts as well as actual land and buildings etc) by husband and wife/civil partner,unless the actual real ownership of the property and the income from it are held in a different split other than 50:50. If so then both partners can make a declaration/election on form 17 for each to be taxed on their actual share. It should be noted that couples do not have an option to have jointly held property and the income from it, taxed any way they like.
As for the second part of your question, you can not back date the election. The election will take effect to income from the date both parties sign the form 17 PROVIDED..the form 17 reaches the Inspector within 60 days of the date it was signed.
It should also be noted that you will not be able to make an election on form 17 for joint bank/building society accounts held in the UK as each spouse is considered to own the whole account. If you wanted to mkae an election for such accounts then you would need to change ownership leaglly by way of a deed or some other legal instrument whic HMRC would need to see.
These are the basics and a lot more detail can be found in the Independant Taxation Manual IN115 et seq which can be found on the HMRC website. Just type IN115 in the search box.
