Worcester Digistat Optimiser DT10RF - error code

[Andre]

Hi I have a 2009 DT10RF and we have just had to change the batteries and having done so we get what appears to be an error code on the transmitter which says EI5 or E15. Worcester told me I have to replace the transmitter which is going to be £140+. Anyone any ideas what the code means and can it be cheaply and easily corrected? I've taken the back off it, nothing obvious, and the 3v backup(?)battery appears full power.

Cheers