Apart from the obvious e.g cleanliness of premises and the attitude and friendliness of staff, I would ask to see a copy of their latest OFSTED report. All nurseries are required by law to have one and this will tell you whether the nursery was found to be outstanding, good or unsatisfactory when they had the inspection.
Actually, you can view these reports online if you wish, on the OFSTED website.
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Old aunts used to come up to me at weddings, poking me in the ribs, cackling and telling me
'You'll be next.' They stopped when I started doing the same to them at funerals.