You are not currently required to hold a valid class rating I the UK/Ireland, some member states require it but not all. This may however come into force in the future.
The retraining requirements will depend on your ability and how rusty you are, any ATO that tells you that you must redo the training programme is trying to rob you. There should be an assessment of your history and recent experience, an individual training programme designed and if you are ready for prof check before that then you should be recommended.
Twin time is very expensive now, best get some FSTD time in!