I called the 0844 number yesterday afternoon and again just now, and at both times it went straight to a voicemail service without any ringing tone. The message is voiced by Derek Fields.
The prefix 0844 500 is allocated to
FleXtel which (assuming that it hasn't been ported away) suggests that the voicemail service is provided by it.
FleXtel Voicemail, like many similar services, sends the message to the recipient as a sound attachment to an e-mail.
With this in mind, perhaps you could e-mail Mr Fields direct on
membership@middlesexam.org.uk