How can an adult get citizenship if he has a British mother?

[CliffP]

This is complicated and any advice is urgently sought:

My partner came to the UK on a working visa with her first husband and child. They had another child in the UK in 1989 but left soon after.
The children were later taken from her by her husband to another country.
In 2000 she came back to the UK and got ILR.
She also applied for the two children that lived overseas to get ILR but never heard anything back.
In 2006 when she applied for citizenship she included the UK born son, who was still under 18.
The application was delayed as the law changed and she needed to take a Life in the UK test and after she passed, she applied again in 2007.
This time it appears she did not include the child - possibly beacuse the UKBA advised her over the phone she couldn't - or possibly in error.
She became a citizen when one son was still under 18 and later, as adults, her two sons became re-united with her.
They visit her in the UK every year from Bosna but only on holiday visas.

Can either son claim citizenship? Legal advice we sought in 2010 said no, legal advice we sought in 2011 said yes for the one born in the UK, but now the UKBA are saying no over the phone.

They have invited us to write in for clarification. Is there any mileage in the youngest son applying to register from Bosnia? Or us writing to the Home Secretary?

Advice on the UKBA website talks of adults registering if they have a British parent but doesn't explain how or which form to use.