The National High Tech Crime Unit (NHTCU) is part of UK law enforcement via the national crime squad. The NHTCU are available to investigate high tech crimes that target or originate from the UK. If you suspect that your computer systems have been compromised then it's well worth contacting the NHTCU as they can help you investigate and potentially prosecute the attacker.
Clearly many businesses would wish to avoid the publicity involved with a compromise - one of the benefits of involving the NHTCU is that they can work on a confidential basis and even if you don't wish to litigate they can use the investigation to help piece together other instances of similar crimes. Another good thing about involving the NHTCU is that they won't require you to interrupt your business during an investigation - they understand the imperitivies of maintaining business as ususal.
There is a handy frequently asked questions section on their website which is helpful to consumers and businesses alike.
It's well worth browsing their website to find out more about ways they can help you.
Monty Paul Durley's blog cites an example of the NHTCU in action to deal with a Phishing attack.