That said, do I need to undertake all PPL training again if I want to fly a 30 year old 152 / Pa28
Of course not. You will just have to take refresher training at an ATO to renew the SEP Class Rating. The amount of training is at the discretion of the ATO depending upon how long the SEP rating has been expired. There are various recommendations in the Regulation concerning the minimum amount of training but these are only recommendations and, at worst, would only comprise a few hours for an SEP rating. If you think that one ATO is asking you to do too much, you can always shop around for another, anywhere in the EU.