I was searching around and found a converter on thebay. This particular piece of kit resembles a mobile phone and simply sits in the cradle and patches your bluetooth phone through the existing kit.
I'll update when it arrives and I've had a read of the manual and tried it.
It seems a simple way to upgrade an outdated piece of kit....no wiring or ripping out to do and full access to the built in mic and radio mute etc. . . I believe it doesn't require a battery either (wish I could say the same for the overrated Blackberry!).
Cost £59.95 inc postage.
from ...
sales@totalcarkits.co.uk
and on ebay as...
item #260577806583 Nokia 6310i CARK91 Bluetooth Adapter Kit CARK 91 CARK11
and looks like...
Even I can hit those buttons...(wish I could say the same for the overrated Blackberry!).

...also, found this review before clicking 'buy now'.
"I've just bought this device from ebay but wanted to review it because it's been excellent. Plug'n'play, I put it in the handset holder, turned it on, and then I set it up with my Nokia phone on bluetooth all within the space of maybe 20 seconds. Now it's paired with my phone, I don't need to do the setup everytime, I just turn it on and the two devices connect."
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