It's important to keep business records such as invoices, receipts and bank statement for at least 6 years. However, HRMC now accept scanned / electronic copies of these documents as long as they "represent a complete and unaltered copy of the underlying paper document." Source HMRC
If you choose to only keep scanned copies please bare in mind that you need a suitable method to store them such that they are not lost in the event of a computer 'mishap'! Think stolen laptop or damaged hard-drive...
One solution is to use an on-line storage facility such as DropBox with 2GB of free storage space.
Finally, Tempo is building the facility for you to store scanned expense receipts within our on-line system... that's all we can say for now but watch this space.