right lets keep it simple [ yeath yeath i know ] if i earned 500 pound every week [ bar 2 ] that would be 25 grand a year , i can earn lets say 9000 , so that leaves my taxable income at 16000 and if i spent 6 grand a year on tools , diesal etc that would make my taxable income 10 grand , so 20 percent of that would be 2 grand ,so my tax for that year would be 2 thousand pounds , is that more or less it or am i wrong [ i often am ]

25000
less 9000 tax free amount (not sure its this high but using your figures)
less 6000 on tools / fuel etc
so your £10000 .... less some suggestions for you

less £500

trips to bank by missus in her car @ 45p per mile
less £480

use of room for office in house gas eleccy etc
less £150

washing of uniform
less £ van depreciation at 20% per year on a reducing balance unless you want to wack it thru in 1 go under your AIA (annual investment allowance) see link .... however if you use the AIA and show a loss this year then you can get a refund on any tax you paid last year OR you can use a loss this year to offset any tax to be paid on profit for next year (which may be handy as if you have to pay tax amount over £1000 then you move into the area of paying on account i.e. end of jan you will pay tax for last year, 50% on account for the preceeding year and the other 50% will be payable in June or July ... cant recall right now personally I took the route of 1 in the hand now so wrote van and equipment off and got a refund.
DISCLAIMER

any of this I have typed may be wrong as I merely clean windows so either take a chance or get an accountant