score:2
- Should I use an immigration service (based in UK) to do this?
It's your choice. In your circumstances I think the application is quite straightforward so I don't think it's necessary.
- We have his mothers full birth certificate (Obtained from the register office) and UK passport - and our Son's full birth certificate with both parent details on it - including passport number of his mother. Do we need anything else? The UK site isn't clear on documents
Technically this should be sufficient because it proves his mother is a British citizen otherwise than by descent (since she was born in the UK), and hence he must also be British.
- If we do the application ourselves (It seems straight forward?) - there is a question about how long the applicant has stayed at an address - if we enter 11 months - it asks for previous addresses - we don't have any previous address for him of course. Can I enter 2 years and then explain it in the additional info section?
These forms for some reason don't account for young children. I agree you should enter 2 years then explain.
- The application forms ask for an address; do we need to provide a residential address or can we provide a hotel where we will be staying?
This should be your home address (ie, the place you're living outside the UK).
- Do I need to submit financial statements? My wife has a UK bank account with money we transferred to it recently for our trip - we can show the source of funds from our home country to her account. - If we need financial statements; would this account be fine?
No. You're not applying for a visa. Your son is British, this isn't dependent on how much money he has.
- Should I include our marriage certificate?
I don't see how it matters but it wouldn't hurt.