Corporation tax - loss carry back with electronic filing

 I'm about to file a return involving a loss carry back.  Pre-electronic filing I'd have submitted the return, an amended prior year return, and my covering letter would have referred to the carry back and repayment due.

What am I expected to do under the new system?  The PY return is not on an electronically fileable format (although I suppose it wouldn't be a lot of work to do it.)  Do I merely submit both returns electronically showing the carry back or do I need to file this year's return electronically and send the amended PY return with a claim by normal post?