Imprisonment is likely but not automatic. The court would consider a number of factors, including the severity of the second offence and its similarity (if any) to the original offence.
For example, if the suspended sentence was imposed for an offence of violence, a similar offence would almost certainly result in the suspended sentence being activated.
If the second offence was theoretically imprisonable but minor and unrelated (e.g. shoplifting a Mars bar), it's less likely that that the suspended sentence would be activated.
If the second offence was non-imprisonable (e.g. a minor motoring offence) it's very unlikely that the suspended sentence would be activated.
See 'C. Breaches', here:
http://www.wikicrimeline.co.uk/index.php?title =Suspended_sentences
Chris