Whilst it depends on who you use, I believe that generally speaking it is 5%, or 10% for positions held overnight. So with the FTSE at 4000, the exposure at £10 per point will be £40,000. So 10% will require £4000 margin for positions held overnight, and £2,000 for intraday positions. Some brokers require an additional 25% tho' - e.g. £5,000/£2,500.