Oh dear, what a lot of disinformation.
ILSNDBVOR is quite correct. Provided that the rating has lapsed by less than 5 years you need to undertake.... "sufficient ground and flight refresher training with an AFIC instructor to reach test standard, followed by Flight and Ground tests with an FIE". (CAP53, Part 1, Appendix H, para 2.14.7)
These rules apply until 1 January 2000 after which revalidation will be in accordance with JAR-FCL1 which are not quite as fellaX has stated them. The experience requirements are for ..."100 hours instruction during the period of validity of the rating including at least 30 hours of flight instruction within the 12 months preceding the expiry date of the FI rating; 10 hours of this 30 hours shall be instruction for an IR if the privileges to instruct for IR are to be revalidated." (AIC105/1998 para 6.1 (a))
A Flight Instructor Rating issued under JAR-FCL1 (not a UK Instructor Rating) will be valid for 3 years, not 2 (JAR-FCL1 1.315)