Firstly, thank you all for your input.
Think the subject of bigamy came up in a post fairly recently. As far as I know the marriage was considered not to have legally existed and therefore did not have to be annulled.
There has been some considerable traffic on the subject it seems, but, mostly appears to relate to personal family stories without actually answering the question.
Although the marriage was null and void, I believe the marriage record remains in situ.
The birth registration would remain unchanged, the fact that the parents were not legally married, does not change the father's paternity.
Jebber
This, Jebber, agrees with my initial feelings on the subject, but, as usual, the more I thought about it the more doubt was raised.
Its a small world Wombat... 
My guess is that your queries could be answered by ordering copies of these Certs...but I would also hazard a guess that no alterations were made...
Romilly.
Always a pleasure to talk to you Romilly. I do, in fact, have a copy of the relevant certificates and they show that all appears normal. But, as mentioned above, a nagging doubt existed.
This raises a supplementary question;
if the bigamous partner wishes to remarry (assuming divorce from the partner of the first marriage) would the existing, and unaltered, marriage certificate from the bigamous marriage render the partner liable to any possible legal proceedings (real or imaginary)? I do hope that makes sense?
Regards
The Wombat