[Solved] checkout success page is blank

I am using version 1.5.1.3
My checkout success page is completely blank. I have tested this with PayPal and also using free checkout. Just goes to a blank page.
Was working earlier in my set-up process, may have changed my PayPal account settings since then in accordance with the instructions under "documentation", but surely this wouldn't have affected the page after using free checkout?

Anyone any ideas on how I can fix this?
Thanks,
Chris.