In reply to John Simpson:
When I put together my first plan I spent a lot of time asking similar questions to you - I'm now on Version 9 so don't worry too much, a year after writing your first business plan will make you laugh anyway!
Good points on the statements above, using 13% on top of employment costs for tax and NI is good, VAT is only relevant for cashflow - this is really important if you're doing some form of capital works/initial investment, if your services are all VATable then roughly accounting for 20% of your profits should cover what you have to pay out - budget for that quarterly in the plan and double check that doesn't send you into the red. The balance sheet is the most critical part of my plan, as we're reinvesting in a rolling programme of capital works - we need to know there is X amount in the bank at the end of year 1 to pay for the growth in year 2. A P&L account won't tell you that. Once you've done it ask your accountant how far off the mark you are. If you're including corporation tax in your business plan you probably want to rethink, as you're probably better showing how you're going to reinvest your first few years profits into growth.
Having said all that, despite what the banks and Alan sugar will say it's not all about the headline numbers, you're probably just as well forming the fuzzy front end of the business plan (marketing mix, 3 year strategy plan and meet the team) and asking the accountants for advice from that, they're there to help you.
And if you're like me and you're a wantrepreneur type - try finding a business partner who is the CFO type to deal with that side of things, you'll end up with headaches if you try to do everything...