If you have misplaced some of the documents, that isn't an issue, as long as you explain the reason why in a letter.
Having said that, if you are using the period October 2018 to October 2020, your documents need to be from the following months, spaced every 4 months:
Oct 2018
Feb 2019
June 2019
Oct 2019
Feb 2020
June 2020
Oct 2020
But at the moment, you only have documents from the following months:
Oct 2020: Joint Council tax letter
Oct 2019 - Joint Bank Statement (Both)
Do you not have any mail at all from February and June of each year?
If not, you might be able to get away with using the following instead, but the reason for the incorrect months and uneven spacing would need to be explained:
July 20 - Bank letters (Both)
January 20 - Bank Letters for each of us (Both) X
July 19 - Joint Bank Statement (Both)
However, using those would only give you 2 sources (bank and council tax) and not the required 3 sources.
The second thing is the fact we recently bought our flat in February. We don't have any mortgage statements but we have a copy of our mortgage agreement and paperwork from our solicitor and title/register deeds from the Scottish Register with our names as evidence of ownership. Should this suffice as an alternative to the mortgage statement?
That's fine. You just need to include:
- Land Registry
- mortgage agreement
- latest council tax statement