Gaz,
I'm not going to be much help here and give you a definitive answer, but I think you're talking about CAPITAL ALLOWANCES.
A Capital Allowance is deducted from your tax over three years.
I think there's some sort of threshold; for example if you fork out £5000 for a WFP system, this would be claimed back over three years.
However, if you've forked out £500 for your WFP system, you'd just claim it from your turnover during the tax year it was purchased in.
The library do a good book called, 'Self assessment for the self employed' which explains all this, but it's been a while since I've read it.
Personally, I just use my common sense and my turnover and circumstances isn't complex enough to warrant an accountant.
For example I reversed into a car earlier this week and caused £200's worth of damage to another vehicle.
I was 'on duty' during the incident, therefore will get a receipt, and will class that accident as an expenditure against my tax bill.
I'm not sure if this is correct, but I'm going to do it; since I was genuinely at work when the accident happened.