Again, as per the legal opinion issued by the Office of Chief Counsel for FAA quoted above, "...the FAA has not considered the absence of a rating as a restriction on a person's pilot certificate." So the revalidation of the type ratings in the 'parent' license, via a proficiency check, is therefore irrelevant to the validity of the 61.75 'piggy back' license. The requirement is only that the license itself remains valid (which, being not time limited is not hard to achieve) and that a valid medical supports the license (see note below). So an EASA type rating that has not been revalidated does not constitute a limitation on the EASA license such that it limits the validity of the 61.75 certificate based upon it. Or, to answer Claudia's question directly - ("do i need to keep the EASA LPC up to date in order to keep 61.75 Ticket valid to fly an N reg squirrel in the UK."). No, you do not.
However, to keep your FAA 61.75 certificate valid you do need to ensure that:
a) your logbook endorsement from a U.S. qualified examiner needs to be re-validated every 24 months.
b) you hold an appropriate FAA medical certificate or 'a medical license issued by the country that issued the person's foreign pilot license'.
This second requirement remains, despite the recent introduction of 'self-certification' for some pilot/license/aircraft types in the UK. So, from a medical point of view, the validity requirements of your 61.75 certificate may now be more onerous than for your EASA license.