I'm not sure if this is of any help, Mr Gazza, but Bevel Heaven list a special tool for inserting the replacement seal:
http://www.bevelheaven.com/stuff-for-sale-brembo.htm
They describe the seal as a "crush washer", which suggests it might just be a tight interference fit rather than located in a groove. Can you get a right-angled engineer's pick behind the washer to 'hoik' it out?