as far as im aware the cost of a course to be undertaken cant be offset against tax, this is cos u cannot gain a new skill or upskill at the taxmans expense, they will however pay for the cost of an additional course, ie sparky going from part 2 to 17th edition as its a neccessary qualification, best bet is to ask tax man, some peeps simply add the course cost into the final accounts if they have to do the short self assessment, if big assessment then all is itemised.
if you start say on 1st march 2010 then you have 3 mnths to tell taxman your start date, anything bought or intended for business use can be claimed from this date, for things purchased prior to start date then only the current market value can be claimed.
example
i bought A satnav for 200 in sept, i went self employed in dec, by the time dec came sat nav was 150, divide 150 by 12 then multiply by the remaining months to get the actual cost you can claim back i got 9mths worth back
as for going back 6 years then if i bought a pc 6 years ago costing a grand and a half the same pc would be worthless at todays rate
capitol expenses are 50 grand a year on a pro rata basis, if you start in april and end in april you can spend upto 50 grand, if you start in sept then its 25grand
andy