I really appreciate your response KFDancer. It made me realise that quite a few of my letters of correspondence did not actually have dates so I have re-thought the whole thing.
Here is what I have now (it's possibly too much but I thought it was better to be safe than sorry!)
March 2016 - Council Tax Bill - Joint
June 2016 - HMRC Letter - Sponsor
June 2016 - HMRC Letter - Applicant
September 2016 - Amended Mortgage Offer from Halifax - Joint
September 2016 - Amex Credit Card Statement - Sponsor
September 2016 - HMRC Letter - Applicant
(I doubled up here because the Mortgage Offer looks so much like a print-out)
November 2016 - Water Bill (Shows our move from our prior house to current house)- Joint
December 2016 - Amex Credit Card Statement - Sponsor
December 2016 - Natwest Credit Card Statement - Applicant
March 2017 - Council Tax Bill - Joint
July 2017 - Amex Credit Card Statement - Sponsor
July 2017 - Natwest Credit Card Statement - Applicant
November 2017 - Annual Mortgage Statement from Halifax - Joint
March 2018 - Council Tax Bill - Joint
Finally, one more question: In terms of accommodation, do I need provide documentation about where we live? We have a mortgage (which I hope is obvious from the correspondence) but I don't know what other evidence we should provide. Also, we lived in rented accommodation for the first year of the period we are relying on (moved November 2016) - Do I need a letter from the letting agent about this?
Thank you SO much x
Out of the correspondence you listed there, I would probably do it like this for somewhat even spacing:
March 2016 - Council Tax Bill - Joint
June 2016 - HMRC Letter - Sponsor
June 2016 - HMRC Letter - Applicant
November 2016 - Water Bill (Shows our move from our prior house to current house)- Joint
March 2017 - Council Tax Bill - Joint
July 2017 - Amex Credit Card Statement - Sponsor
July 2017 - Natwest Credit Card Statement - Applicant
November 2017 - Annual Mortgage Statement from Halifax - Joint
March 2018 - Council Tax Bill - Joint
As for proof of accommodation, you'll need the mortgage statement and the Land Registry entry, which can be purchased online for £3 and printed off. No need to include anything from the old letting agent/landlord. We moved from rented accommodation into our own house around the same time. We only had the mortgage statement and land registry document, and that was fine.
To get the land registry document,
search for your house here, and on the next page, purchase and print the title register for £3.