If you run through Google 'westland BERP' you will get a number of results [including ...
http://www.whl.co.uk/history_timeline4.cfm ] that refer to the BERP being developed and introduced.
Were you perhaps after tecnical material? That I would expect to be commercial confidential even now.
The Putnam book on Westland by Derek N James has a fair bit on the BERP but again lots of images, general references and a timeline but no hard and fast technical stuff to speak of.