I'm in touch with HMRC about this. I have bank statements and receipts for some but not all expenses pre-2010 but an agent managed the property thereafter. I calculate I made a net loss up until year ending 5th April 2012. The administrative work including responding to agent queries regarding maintenance etc probably takes up a few hours a month. Before the agent took over in 2010 it took up a bit more time as I had to go and sort things out and spend much more time dealing with repairers etc. I maybe averaged 4-5 hours of my time per month averaged over the year.
I have also had to travel to the property which is in Sheffield (I live in Norwich) so I have estimated up to three journeys a year for mileage before the agent took over management. So I guess the time spent actively managing the property by physically going there versus the time sat at home on the phone to agents (hence use of home as office) has to be kept distinct? Sorry for follow-up question but I hope it helps clarify the issue