SA302 is spot on, for me I ended having to pay more tax. I keep a little extra in my limited company for a rainy day. Its better to keep it there as if you take it out you pay more tax.
But, as I need to show a decent amount of earnings I had to take more out and pay more tax. I paid myself and my wife more and we both had more earnings in the 40% tax bracket. (we paid us a bonus 🙂 )
They asked my accountant to fill out a form, this sort of just asked them what the earning predictions were.
I went via a broker and the fees were very small, the broker knew the my accountant so it made things run very smooth (HMRC were slow), the broker also worked close with the mortgage team so any niggles were sorted.
I would recommended a broker but I would ask you accountant if he/she works with any.
Even with all the above, it was a proper pain with the HMRC as they are not setup to be involved in the process. We had to get tax returns submitted and then get a SA302 days after, we had to phone the HMRC and ask them nicely to FAX over the SA302.