I am in the process of getting things in place to start my own business and have a number of items already that I can use which were bought a few years ago. Can I use these (to save on paying out again for something already in my possession) and claim them back against my income tax in my first year trading? As they were bought about 3 years ago (before I considered turning a hobby into a business) I no longer have receipts for these items. Can I still claim the expense back?

Have a look in the HMRC Business Income Manual at pages BIM46351 and BIM46355:

• http://www.hmrc.gov.uk/manuals/bimmanual/bim46351.htm

• http://www.hmrc.gov.uk/manuals/bimmanual/bim46355.htm

You can see that if revenue expenditure is incurred within seven years of starting the trade, it can be claimed on the day trading started, if it would have been allowable if incurred after trading had started. However it seems to me that you are asking about capital expenditure, which is specifically excluded from this treatment, if you look there. It appears to me that the correct treatment is to calculate the market value of the assets on the day you started trading and introduced them into your business (which may be somewhat less than three years ago when you originally purchased them), and calculate capital allowances accordingly. 

If you are asking about revenue expenditure, I would still use market value of the items on the day you started trading. This situation is not the same, but nevertheless very similar, to that which can be found in the HMRC Employment Income Manual pages EIM21660 and EIM21661:

• http://www.hmrc.gov.uk/manuals/eimanual/eim21660.htm

• http://www.hmrc.gov.uk/manuals/eimanual/eim21661.htm

Here it explains that if the taxpayer charges the business more than the market value for the introduction of an asset then the taxpayer is subject to income tax on the difference.