Employment letter for overtime

[Daisy2015]

Hi all, I am about to get my employment letter from the personnel manager for my fiancee's spouse visa. However, Im a bit confused about the general gross salary.

Basically, I have to use cat A and my gross per month for last 6 months (from sep 2015 to feb 2016) is just over £1700 including overtime but in sep, I was contracted as part time 16 hours + unlimited overtime and the basic pay was about £650 per month and the rest £1150 was overtime.

From oct, my manager managed to get me in full time and I had a new full time contract which was 31 hours + unlimited overtime so since my contract has been changed, the basic pay also has gone up to £1000 and plus £700 overtime. I still worked the same hours when I was both on part time and full time so technically, the contracted hours don't really matter.

My question is, when my personnel manager put gross salary down on the letter, does it mean basic pay? Or the total monthly gross? Im confused with this. Does she need to take the lowest basic pay out of the 6 months which is going to be £650 and multiplied by 12? Or is it like the total gross per month which is about £1700 multiplied by 12?

It won't be so much problem if she can just put £20400 (£1700*12) as a gross salary. If I can use this way, would it be better if she put it down as this amount is including overtime or not mention anything and put £20400?
However, if I need to use only the basic pay as gross salary, it will only be £7800 per year plus the overtime £12600.
Also, if I use only the basic salary, I think it may be a problem to prove my 6 months wages as my contract got changed in oct and my miss needs to apply for the visa in 2 weeks, it's been only 5 months since I got the full time contract even though I have the same amount gross throughout the last 6 months. Which one do I need to use? And I actually get paid 13 times not 12 times a year as I get paid 4 weekly.
Should I add this in the letter as well? Another question is, I don't have such a contract as they do everything with computer. It says my contract hours, start date of working etc on the computer but I don't have a paper contract. Do I need to get a paper contract or can my manager write it down on the letter that we don't get a paper one anymore?

Last question is i have been working for the company since 2004 but because I had to work more overtime to meet the requirement, I started getting paid over £1550 from july 2015 so can I still use 6 months payslip even if it's not a new job or new company? If anyone can answer to the questions, it will be really helpful.
Thanks