In reply to Game of Conkers: The tax code works reasonably well for individuals who fall into the 40% tax bracket and have total incomes less than 100,000. The only things you need to do there are ensure they tax you correctly on extra income (interest and dividends mainly) which are taxed at basic rate and you have to tell them how much you get each year so they can adjust teh next years tax code (you can minimise this admin and tax issue if you have a standard tax rate partner and just make sure all savings and investments are in their name... thats totally legitimate tax avoidance, not illegal evasion). Also, if you give to charity regularly plus gift aid sponsorship that annual amount can reduce your tax code for the next year. Its a tax payers responsibility (technically) to make sure the tax code is correct and includes correct adjustments for these sorts of things (i.e the bits teh tax man doesn't know about). If ou fll into teh £40 to £100k bracket they normaly don't send a self assesment and just expect you to phone up each year and get your tax code adjusted for each of these items
Where it gets more complicated (but they do send self assesments automatically as soonas they realise you are in this bracket) is that if you earn over £100k they need to reduce your tax free allowance by £1 for each £2 over £100,000 you earn until you have a zero tax free allowance. If you then need taxed a bit more on other stuff (work related perks, interest,dividends etc) you end up with a negative tax code.
If your income varies a bit (say a salary just under 100k and a bonus that might be 10 to 20k ish) their 'estimate' of your annual income may well be incorrect. If you are likely to fall into an annual income of between 100 and about 117k (i.e. 100k plus 2 times the standard tax free allowance) you need to keep a close eye on things and make sure their estimate is as accurate as it can be and be aware that if your income moves up or down (i.e. a good or bad bonus year) you will have over or under paid tax within that tax year.