In that case a service manual would be your only option. I have checked my usual sources but unless you know the real manufacturer (PB is a badge for dixons grp) it will be very hard to track down. Might be worth dropping an email to there tech support dept or even buying a new one at £60 inc vat atm probably wont be worth repairing. Check this link
Standard practise is to replace the inverter board if you can source one. Usually overheated/burnt transformers on the inverter are the cause of no backlight at all. If the backlight comes on at the start and then goes off try replacing the electrolytic capacitors on the inverter board. Ive found this fixes 70% of inverter board problems. Good luck.