Could you tell me how I can achieve the correct humidity when maturing hard cheddar type cheese? How can I measure humidity?

Humidity control is difficult on a small scale. Hard rind cheese need about 80% humidity, in a fridge the humidity is probably less than 50%. An additional issue is the air movement, fridges tend to have rapid air movement, because of the fan, and this tends to crack the cheese. Probably best to put the cheese on a plate, standing in a larger dish. Put water in the dish, and drape a damp tea towel over the cheese so the ends are in the water. Turn the cheese twice weekly, and wipe off any mould which forms. A thermohygrometer measures temperature and humidity, a small battery operated one is about £25 from laboratory suppliers.