As usual there seems to be a lot of advice being given, some of which is going to hinder rather than help you. I'm self employed so I actually know what's involved.
1. You can manage your finances however works best for you. You don't have to use computers or any spreadsheets / software, but you will need to keep all your receipts and paperwork somewhere safe. You will need these later.
2. You don't have to use a dedicated business bank account. This was one of the first questions I asked my accountant when I started. You can usually use a personal account but obviously only if you are trading under your actual name (e.g. Pete Smith), and as long as you don't pay in more cheques per month than the account allows. Business accounts generally pay less interest and charge more for the privilege. My accountant worked at the Inland Revenue for years before branching out on his own, so if he says it's fine that's good enough for me. Whether the banks would agree is another matter.
3. You don't need to worry about what taxes you will pay yet. All you need to do from a tax point of view is contact the Inland Revenue (now called HM Revenue & Customs) and tell them what you are doing, within 3 months of starting up. If you miss the 3 month period you pay a £100 fine.
They will ask a few questions such as trading name, nature of business, address and a few more. This will take no more than a few minutes. They would usually then arrange for Class 2 National Insurance contributions to be collected from your bank account. However, the fact that you are going to be employed and self employed at the same time means they will have advise you on your NI contributions. You will already be paying through your other employment so Class 2 might not be appropriate in your case. Contrary to popular belief, they are very helpful and friendly on the phone so when you are ready to register just call them and ask away.
4. You don't need an accountant, but I would advise that you use one. The first few years are a bit of a nightmare as they collect half of the tax from the upcoming year, based on profit from the previous year. Your accountants fees are tax deductable anyway. It sounds easy enough to do your accounts yourself, but you will probably have some items (like hardware) which will be treated as capital allowances and these can't just be deducted from your taxable income like items such as stationary can. You get 40% of the purchase price deducted for depreciation in the first year, then 25% of the balance deducted for each of the next two years. Computers I believe were allowed at 100% in previous years, but now I think this has stopped.
5. Public liability insurance is required. I have £5million PL and it costs me around £120 a year. Don't work without it. I think it's actually illegal to be self employed and not have PL insurance, but don't quote me on that.
6. Not sure about this one. I guess your contract with your employer might stipulate certain things about being employed elsewhere. You would have to read it and see.
I can't possibly see why you would need to see a solicitor. You just need to focus on keeping all your receipts for things you buy, and make sure you have copies of all invoices you get paid on etc.
You have to keep all paperwork for 5 years just incase the I.R. decide to come and check up on you in the future.
Also, any savings accounts, bank accounts, gifts to charity etc all need to be taken into account when working out your profit / losses at the end of the tax year.
Mini Cash ISAs are a great way to legally hide money from the taxman. He doesn't need to be informed about any ISA that you have, and any interest accrued from that ISA is tax free.
In your first tax year, you might make a loss anyway. You are allowed to earn £4745 per tax year (this amount might have changed slightly now) before you get taxed. Over this amount, the first £2000 odd is taxed at 10%, then the next £29,000 odd is taxed at 22%, then everything above that is taxed at 40%. Then you would also get Class 4 NI contributions taken, based on your income. I think this is a futher 8% of your taxable income.
Basically a lot goes to Gordon Brown, which is why so many people (not me) do as much work as possible on the side for cash, and on the books earn about £10,000 which means an annual tax bill of around £1000.