Redo logs not being applied to standby - "Fetching gap sequence in thread 1" in standby alert log

We have a standby database, that was all working fine until last Thursday.  redo logs were all happily being applied.

I've not got to the bottom of why, but one of the old redo logs didn't apply, and then the redo logs were deleted from both production and standby, and so no further logs have been applied.

As the standby was several days out of date, I've recreated the standby by taking an rman hot backup of production, and then restoring to standby, then placing the standby back into standby mode using

alter database recover managed standby database disconnect from session;