I make it £3268.50. Here's how:
Income £24100
Allowable expenses £6950
Total Income £17150
Personal allowance = about £4900
Total taxable amount = £12250
First £2000 at 10% = £200
Next £10050 at 22% = £2211
National Insurance at 7% = £857.50
Total tax payable = £3268.50
First payment on account due January 31st 2008 = £1634.25 + any tax owed due to earning more this tax year than last tax year,
ie, if you had a total taxable amount of just £10,000 last year you would need to pay the tax owed on £2250 (difference between this year's total taxable amount and last years) which would be £652.50 and your first payment on account would be £2286.75 with a second payment of £1634.25 on July 31st 2008.
The only way of reducing this amount honestly is to examine your allowable expenses and find ways of claiming more, such as so much per week for use of home office, stationary, washing machine, power, storage, telephone use, water rates, uniform, computer, etc. Check with an accountant or even better book an appointment with your local tax office and ask them what you can claim that you are not already claiming, they are duty bound to help you.
The other point to bear in mind is that your £1200 for your system will include some consumable expenses that can be claimed as 100% deductible. Items such as Poles (often last less than a year), hoses, filters, resin beads, connectors (most need replacing every 6 months), brushes.