[Exim] Data time ACL not working

I have the following two 'deny' ACLs:

  deny  condition = ${if match{$header_subject:}{\Nindows-1251\N} {yes}{no}}
        log_message = SPAMFILTER: Junk in subject
        message = Go away spammer

  deny  condition = ${if match{$header_subject:}{\N=\N} {yes}{no}}
        condition = ${if match{$header_subject:}{\N\?\N} {yes}{no}}
        condition = ${if match{$header_subject:}{\Nindows\N} {yes}{no}}
        log_message = SPAMFILTER: Junk in subject
        message = Go away spammer

that I'm trying to use to stop messages that have some variation of:

=?windows-1251?B?VHB54W/j6OFv9+37ZSBj8mHt6ug=?=

in the subject.

But it's not working. Can anyone see anything wrong with the ACLs?

Elliot