Thanks for your question.
I'm afraid that if you did not have your CAS letter and submitting this before your visa expired then you are an overstayer. Though I appreciate that you were not able to obtain your letter because of the festive break, the UKBA are not under any obligation to give you any flexibility on this. They are permitted under immigration rules to refuse on this basis.
If you apply to extend your visa once you have your CAS letter then you would have to plead that they exercise their discretion and not refuse your visa under the ground of any overstay. You should explain in detail your personal circumstances and difficulties which led to you not acquiring your letter in time.
Their position would be that it was your responsibility to take in to account the festive period and breaks in the administrative staff at the university and request your CAS earlier accordingly. I have to say that this is a reasonable position to take but you may get some luck if you have an understanding immigration officer and your application has not other defects.
If you are unsuccessful and are able to get a doctor's note stating that you wife is not medically able to travel because of her pregnancy then you should be granted further leave to remain until she has had the child and is well enough to travel again. .
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