Hi Peter, can you advise of the exact message displayed by the boiler?
If this is in fact a hardware fault, a gas safe engineer will need to attend, once I know the exact fault shown on the boiler I can quote for the replacement part, and should you want me to recommend a local engineer in your area, however would need your postcode.
No problem Peter, its suggests the boiler is still in diagnostic mode hence why you were given reference to the NTC rather than fault code F10-F13.
A new NTC (temp sensor) is around £10, but as this is still under warranty parts should be free of charge.
I would start by contacting the installer to get them to come back and investigate the fault, and see if the repair can be done under warranty, as this would appear to be a hardware failure, and not something which can can be fixed by the user.
Should you wish me to recommend someone to come out please provide me with your postcode.