I remember seeing a definition which defined evasion as lying and avoidance as hiding.
Antony Seely, policy specialist on taxation in the House of Commons Library has written:
In recent years tax avoidance has been the subject of considerable public concern, although there is no statutory definition of what tax avoidance consists of. Tax avoidance is to be distinguished from tax evasion, where someone acts against the law. By contrast tax avoidance is compliant with the law, though aggressive or abusive avoidance, as opposed to simple tax planning, will seek to comply with the letter of the law, but to subvert its purpose.
Moot isn't it but using the words: aggressive, abusive and subvert suggests there is a fine line.