1. Who has told you you can only have one bank account? Was it the Official Receiver, or your bank? If it was the bank I think you can safely ignore it - just open a savings account with another bank and have your housing benefit paid into that, but it may be worth checking with the Official Receiver that they are happy with this. Notes: a) By savings account I do not mean a basic bank account, but the old style savings book account - often operated with a passbook. There are still such accounts into which money can be paid by BACS, so the local authority could pay your housing benefit into it. b) The Official Receiver will be interested from the point of view of making sure you are not accumulating savings, so the account should only be used for the benefit and therefore virtually emptied every time you pay the rent.
2. If the Official Receiver told you there is this limitation I don't understand it. There is nothing I have seen in their Technical Manual which imposes such a restriction.
3. I would not take the money out and keep it in cash. There is always some risk of burglary etc. So - if you really can't have a separate account - leave it in the one you have, but of course you will just have to be disciplined and make sure you don't spend it on other things before the rent is due.
4. Of course, you could always pay the rent early - i.e. as soon as the money goes into the account.