That was the case but that requirement is not in the current CAP 804 which states:
3.5 FRTOL Validity
Stand alone UK FRTOL issued before 17 September 2012, had a ten year validity. When these licences expire, if the holder still requires to hold an FRTOL, they must complete form SRG 1106 and pay the appropriate fee.
A UK FRTOL issued after 17 September 2012, does not expire, but it is not valid unless the holder has language proficiency in English at Level 6, or at Level 4 or 5 that has not expired.
Note there is no mention of the majority case of a FRTOL issued with a pilots licence prior to 17 Sept 2012! As you already have a lifetime FRTOL, it would appear there is now nothing to do!
Also I would need to do a full PPL skills test.
NO NO NO! there has been no such requirement for the past 13 years! PPL Skill Test is for initial licence issue only. You do a Licencing Proficiency Check.