Generally, they want to know if you have a mortgate, how much it is, how much equity you have in your home. Any savings, bonds, loans etc. Any dependants you may have and what your inside leg measurement is (or so it feels like
Do they not have a form for you to fill out? When we were applying, they had specific forms which asked the exact questions they wanted. A letter by you seems a bit wishy washy, you could leave something out quite accidentally or, (and this may be the case) include something which they don't/can't ask, but which would throw a different light on things maybe? Who knows!
But no, no tips, sorry. I think in these instances you can only give the facts, being as open and honest as you can.
Good luck with it all.