Cremeegg are you speaking from personal experience with revenue or your own interpretation of the rules?
The rules quite clearly require that you have invoices for every item, (the precise requirements for an invoice to be acceptable are very detailed, many shop receipts and certainly credit card slips, would not qualify).
The rules also require that you keep documents for 6 years. Which is less time than a capital allowance runs.
It is a self assessment system so that your own interpretation of the rules is all there is until such time as you have a Revenue audit.
I have a number of items in a rental property where I am claiming allowances based on an estimated value when they were first rented. I had photographs of the items in the file, to prove that they actually existed. And ads from done deal to support the value I was using for the second hand sofas etc.
When I had a Revenue audit, this was not raised as an issue.