Thanks for your question.
An interview is not necessarily certian, they may accept or reject without asking for an interview with the applicant (though you would be able to appeal).
If they do decide to interview then he statement made from the UKBA on "Why may I need to attend an Interview" is as follows:-
When a visa officer cannot make an immediate decision about whether you qualify for a visa, you may be asked to attend an interview. We aim to do this within 15 working days from the date the visa officer assesses your application.
Always try to make your application well in advance of your intended travel date.
It is important to note thought that this is an aim, not a duty upon them that you can enforce for breaching it.
The UKBA is frequently very backlogged and it's takes tme to get to and then investigate an appliction. They aim to make a decision on the application within 2-3 months of the application being submitted. Again though, it's very much a moving feast and depends on the amount of work they have at the time of application.
You should (both) keep pestering them by telephone/letter and if more than three weeks go past without hearing from them the you should get a solicitor to write letter on your behalf. If you have already instructed a solicitor then ask him to be pro-active in writing to them.
As long as they have acknowledged receipt of the application it will get dealt with in time (probably fairly soon given the length of time you have waited so far) and though it's very frustrating I would urge patience.
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