28 Interpretation of Part IX.
(1)In this Part of this Act, unless the context otherwise requires,—
“furniture” includes fittings and other articles; and
[F1“premium” includes—
(a)any fine or other like sum;
(b)any other pecuniary consideration in addition to rent; and
(c)any sum paid by way of a deposit, other than one which does not exceed one-sixth of the annual rent and is reasonable in relation to the potential liability in respect of which it is paid.]
(2)For the ....
http://www.legislation.gov.uk/ukpga/1977/42/section/128

pre·mi·um
n.
1. A prize or award.
2. Something offered free or at a reduced price as an inducement to buy something else.
3. A sum of money or bonus paid in addition to a regular price, salary, or other amount.
4. The amount paid, often in addition to the interest, to obtain a loan.
5. The amount paid or payable, often in installments, for an insurance policy.
6. The amount at which something is valued above its par or nominal value, as money or securities.
7. The amount at which a securities option is bought or sold.
8. Payment for training in a trade or profession.
9. An unusual or high value: Employers put a premium on honesty and hard work.
http://www.thefreedictionary.com/premium

The definition from wikipedia is that this is a deposit paid by a buyer to the seller where there is a right to withdraw from the contract. However, on withdrawal by either party, the buyer loses the deposit or the seller has to pay back double

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Note added at 56 mins (2011-03-11 10:28:12 GMT)
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Or non-REFUNDABLE deposit is probably better

my opinion