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Their website https://www.midlandrailway-butterley.co.uk/ just says 'later in 2021'. Although indoor museums can reopen later this month, opening may still be constrained by 'social distancing'; it's not known yet what will be allowed. I appreciate Butterley has large buildings but they may be waiting to see exactly what will be required to reopen. (The small museum I'm involved with, despite an outdoor area, is so confined that we expect we shall not open until things are much more relaxed!)