I'll let the accountants be more precise, but if the work is done in 2013, then to me even if you pay in 2014 it's a cost for 2013. If you take a rent that is late, say due 29th Dec, and paid by tenant on 2nd Jan, as far as I know it goes into the books of 2014 so I assume the same applies to expenses.
Personally I just put the cost in my books as the date of invoice. And ditto for rent.
Mandelbrot will be along shortly to tell me how I'm doing it all wrong