Angel_1_five - If your licence has expired by 20 years then one assumes that it is a UK national one and not JAA. If this is the case then nothing in Part-FCL is relevant as the new Regulations apply only to EASA licences and ratings (JAA licences and ratings will be deemed to be EASA ones after 8 April this year).
If the licence is a 'lifelime' one, issued after 10 Nov 1975, then all that is required is to revalidate the SEP class rating, which involves a Skill Test including an oral theoretical knowledge test by the examiner (this is theoretical knowledge related only to the class rating). If the licence was issued before 10 Nov 1975 it cannot be renewed and you will have to obtain a JAA licence, which will mean passing all of the JAA theoretical knowledge examinations.
In all cases, you will also need a valid Class 1 or 2 medical certificate.