Thanks for comments. Ended up phoning Caterham after sales as web site still saying no stock. Turns out web site doesn't log a customer request to advise when stock in. So they wouldn't know if people (such as myself 2 months ago) had been trying to get some.
They don't have BRG or yellow in but will now order some. Should have in 7 days.
Lesson is don't rely on web sites, talk to someone.