axelc,
The qualifying period for ILR is 24 months living in UK
If you arrived to UK 18 October 2009 on spouse visa, then the EARLIEST you may apply is 28 days before the end of the qualifying period.
In your this means you can apply anytime from NOW until the visa expiry date in Decemeber 2011.
Before applying for your ILR you will need to have a pass certificate for Life in UK test.
Concerning the documents for Evidence of cohabitation, the UKBA suggest using :-
..6 letters and/or other documents addressed to you and your partner at the same address as evidence that you have been living together during the past 2 years.
If you are really struggling to provide 6 items covering the past 24 months then you need to give a detailed letter of explanation.
Documents may be addressed to both of you, or to each individual.
Please do try to think of any documents or letters at all that can be used.
Usually there should be something or other. (just for example council tax bill, water bill, electric bill, appointment for NI number, council tax showing you notified them that you are living together, bank statements, notification of debit cards, phone bills, TV licensing, payslips, doctors appointment letters
Try to ask your GP, Priest, family, friends, neighbours or even landlords to provide a simple support letter confirming how long they have known you both as a couple and that you have been living together for the past 24 months.
Additionally I would include a covering letter that you have a child together who was born in the UK, and include a copy of the birth certificate.
You need somehow to ensure that a full 24-month period of co-habitation is evidenced.
Submit any letters you may have received in either joint or single name, you may have more than you think.
I would not advise you to pay the additional cost for personal application at the PEO if you cannot satisfy the documents for cohabitation.
Without the required documentations for cohabitation it is likely that the back-office caseworkers will need to check/confirm your explanations and whatever supporting documents you supply.
So this would mean no decision on the day and no fast-tracking of the application. Means the extra money is not well spent.
