Just to advise, CyBook have made a very fast response (only 3 working days).
They emailed today to advise that "the On/Off button is broken. As the broken parts are not covered by the warranty, we invite you to issue a 20€ payment (parts, labour and VAT included) + 15.10€ for shipment".
I thought this was reasonable and the response was very quick compared to other suppliers. It's good to know that broken parts can be sorted!
When returning your unit for inspection by the supprt team you will need to include a returns form, including your individual 'RMA...' reference code. The payments forms was attached to todays post inspection email.
Overall, I'm very pleased with the outcome and am most grateful for the advice I have received from forum users.
All best, Heth