Spouse Dependent Visa - Need Guidence from Experts

Hi,

I have been living in UK on Working holiday maker visa since Dec 03, 2003 and my visa will expire on 01/12/2005. My wife is on student visa and her visa will expire on 28/02/2006. She will apply extension as her course will finish in other two years.

I want to apply as dependent of my spouse.The gap between my and her visa expiring is almost 3 months. Is it possible that she can apply for her extension before my visa expires, lets see late Nov 05 and include me and my son(my son is dependent of her as well and his visa expire on 28/02/06) and we pay only 500 pounds once

OR i need to apply before as dependent of her for three months and the we need to apply again in Feb 06 for extension together?

Please guide what is best time to apply

Many thanks

regards

Ali