What is the expiry date for using a cinema voucher? Can I use it in exceptional circumstances to reserve a cinema seat for a showing that will take place a few days later?

No, the expiry date of your cinema voucher is the last possible day you can use it to access a showing. If your cinema voucher is not, for example, valid after 19th April, you cannot buy tickets for showings that take place after 19th April. In that case, you will have to use your cinema voucher not later than on 19th April, the cheque's expiry date. Take the initiative. Keep an eye on the dates. Write them down in your diary and set a reminder.