Paul,
It isn't quite that simple. The bond has to be lodged (paid) in instalments. It is paid in cash. The cash may be raised by a commercial secured loan. These loans are in effect by way of a second mortgage (if there already is a first one.) Once the new loan is added to the existing one, the total sum shouldn't normally exceed 60% of the value of the property.
For example if you have a property worth £250,000 with an existing mortgage of £80,000, then the maximum sum that could usually be advanced would be around £70,000 (based on the 60% equity ceiling.) If you already owed £150,000, then that property would not provide security or be acceptable for this type of loan. It would also require that you could afford the new combined loan by way of normal lending criteria restrictions.
Where this isn't possible, a guarantor may be able to provide the necessary surity, however they would assume all the same risks.