Just had it confirmed by Crabtree, yes replacement covers are available for flush fitted consumer units.
Crabtree do not sell any lids or replacement parts for consumer units from mid 90s or early 2000's. This was confirmed by Karen from Crabtree. They do not do any RCBOs either. I did mention that some have been able to do this but the advise given back is that its likely that those who have put compact RCBOs in very old units, are likely to have done other modications which Crabtree can not comment on.