[Exim] Logwrite headache

My .forward file looks like this:

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# Exim filter

logfile $home/mail.log
#logwrite "TEST\n"

save $home/testfile
---------------------------------------------------

... which works fine - writes the incoming email to testfile, so far so
good. However, when I uncomment the logwrite line, not only does it not
write to the logfile (which has full write permissions) but nor does it
write to the testfile.

Testing the filter (with logwrite line uncommented) produces this:

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Testing Exim filter file /path/to/.forward

Filter: start of processing
Logfile /path/to/mail.log
Logwrite "TEST\n"
Save message to: /path/to/testfile
Filter processing ended:
Filtering set up at least one significant delivery or other action.
No other deliveries will occur.
Filter: end of processing
search_tidyup called
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Here are a few lines from /etc/exim.conf

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address_pipe:
driver = pipe
return_output

virtual_address_pipe:
driver = pipe
return_output
user = "${lookup{$domain}lsearch* {/etc/userdomains}{$value}}"
group = nobody

address_file:
driver = appendfile
delivery_date_add
envelope_to_add
return_path_add
---------------------------------------------------

I get the feeling that the logwrite command is disabled somehow but I
can't see anything obvious staring at me in exim.conf