I have just read the manual - the external probe you mention is an external temperature probe fitted outside the premises - but it's optional and you say that you don't have this option.
The Error Code E08 says that there's a fault with something that your boiler does not have so clearly the fault shown is a phantom - the electronics are playing up.
The manual also says that the E08 fault can not be reset.
I think you need a service engineer.
Good Luck..., March 2017
