Have you been into the BV8's SETUP menu and selected a speaker type (TV SETUP, OPTIONS, SOUND ..)  - I think you need to do this in order to activate its POWERLINK sockets.

Yes the Powerlink socket must be active because it is working but the volume is much too low on speaker 2 which is the external speakers. A comfortable listening volume for the TV in built speaker is around 30 but even at 50 the CX's are not properly driven. Perhaps there is a fault with the converter? I see no additional menus. The "centre" option appears in speaker 3 (when both TV and external speakers are in use) but there are no volume controls for the external speakers. 

In that case I can't help.  I thought there may be cross-talk to unactivated PL sockets (hence low volume output), but if you are sure of the MENU setup then it must be something else.

Another thought - has the foam perished on the CX50s?

Not at all.My MX4002 drove them brilliantly but this huge amplifier does nothing. There is no external gain adjust so perhaps I should assume there is a fault with it. It is a 50w/Channel amp but it seems more like 5w/channel

Take a look at Elephant’s post 4 Jan at 9:16 here, as it alludes to similar problems: 

It certainly does, although the comments allude to the BV8 speaker being overpowered rather than the external ones being under-powered. I definitely think its the latter because even with the centre function turned down as far as it will go there is simply no oomph getting to the CX50's they are acting like tweeters with no bass at all. The BV8 sound is very punchy so together on speaker 3 they are not bad, just nowhere near as good as the MX driving them with its own amp. Thanks for your reference and help.

Regards

David