CET ERROR: requested WAL segment has already been removed

Dear all,

Using Postgresql 9.3. on Windows x64.

I had streaming replication set up from master to slave, but the slave
shut down and became out of sync with the master. So... I tried to set
it back up, without success.

I then checked the *master* log and, although the server seems to be
working, the log is full with
"CET ERROR: requested WAL segment 00000001000000090000008E has already
been removed".

What should I do? While I have this error replication fails, because the
slave looks for the same (missing) WAL segment. The master seems to be
working, but of course it's possible that I've lost data somewhere.

Is there anything sensible I can do beyond restoring from the last good
backup?

Thanks
Martin