As the surviving spouse, you have both 'prior rights' and 'legal rights'.
Your prior rights give you:
(i) the house (if applicable), providing that the value does not exceed �300,000. (If it does, you're entitled to �300,000 from the sale of the house) ;
(ii) the furniture (up to a maximum value of �24,000) ;
(iii) all money held in cash, bank accounts or similar funds, plus money from the sale of other assets, up to a maximum of �42,000.
If there's any remaining cash (whether from bank accounts or the sale of assets), your legal rights give you one third. Your husband's children also have legal rights, to share one third of that cash between them. The remaining one third of the cash is distributed as the 'dead's part', which effectively means that it's also shared between your husband's children.
http://www.hmrc.gov.uk/manuals/ihtmanual/IHTM1 2151.htm
Chris