The small companies rate of Corporation Tax is 20% compared to main rate of 23% (2013/14). The small company rate is applied if your profits are below £300k, however, if you have associate companies, the £300k is spread between them equally.
For the purposes of CTA10/S25 (4), formerly ICTA88/S13 (4), a company is an associated company of another at a given time if at that time:
- one of the companies has control of the other, or
- both of the companies are under the control of the same person or persons
http://www.hmrc.gov.uk/manuals/ctmanual/CTM03710.htm
But what some businesses forget is that if you have a subsidiary that has become dormant it stops being associated
an associated company which has not carried on any trade or business at any time during the accounting period is disregarded – if it is an associated company for part only of the accounting period, the rule applies to any time during that part.
http://www.hmrc.gov.uk/manuals/ctmanual/ctm03580.htm
steve@bicknells.net
Business Accountant 
Same applies for larger groups where the threshold for payment of corporation tax in advance by quarterly installments (£1.5m) is divided by number of associated companies and so can be triggered earlier by associated companies. Moving to installments can be punishing from a cash flow perspective as in the first year you pay both last year’s and some of the current year’s tax.