Hi Francis,
The Home Office infact do have many outstanding applications of the same kind and yours may be one of those currently under the backlog of older cases as it has been quite a lengthy period since you havent heard back.
There is no definite answer as to how long an application will pend with the Home Office as it is under their discretion to decide on each individual case.
As a general guidance it is worth sending any application to the Home Office via Recorded Delivery to ensure you have a tracking number for reference.
Ultimately my advice to you would be to request your legal representative (if you have one) to call the Home Office for an update into your case as a matter of urgency as calling them doesnt always resolve the situation. Ensure you send this letter out via recorded delivery and if you desire you could head it 'Letter of complaint'.
If your case falls under the backlog they will inform you; and thereon you will need to wait slightly longer.
Hope that helps :)
Regards