Re GARDAN's post - Position in UK not quite the same as US.
A 'Money Market' deposit carries no more real risk than a retail deposit with the same bank. The bank will quote you a fixed rate for a fixed period based on LIBOR rates and it would take the collapse of the bank itself for your deposit or interest to be compromised. You ought to get a slightly higher rate than with a retail deposit. In practice I have found any such differential to be minimal to negative though, AND - you pay tax on ALL the interest. MM deposits are very simple convenient and easy for shuffling large wads of cash around whilst earning a bit of interest though - or so I have found. If you need some or all of the cash back during the deposit period, there is usually a small penalty + forfeit of some interest.