I don't think that there is any short, simple explantion for this but for great reading on the subject read Rebecca Benneyworth's article over on Aweb.
See here : http://www.accountingweb.co.uk/topic/tax/entertaining-round-rebecca-benneyworth
HTH,
Shaun.
__________________
Shaun
Responses are not meant as a substitute for professional advice. Answers are intended as outline only the advice of a qualified professional with access to all relevant information should be sought before acting on any response given.