I finally managed to get hold of the vendor of the new property and arranged an inspection this afternoon with the contractor and the leasing agent.
My original plan had been to get two quotes:
1.a minimalistic repaint and tidy up;
2.a complete refurbishment.
The condition is actually slightly better than I recalled from previous visits. The whole flat looks a little bit tired (scuff marks on the walls, a couple of cracked wall times in the bathroom etc) but there is nothing inherently wrong with anything. The agent has advised that it could be rented out "as is" without any difficulty. After discussing with the agent, I have concluded that there is no need to get a quote for the minimalist repaint and tidy up.
The complete refurbishment effectively involves gutting the flat. All the existing built in furniture would be removed, the false ceiling would go and the kitchen and bathroom would be completely redone. I decided that there was no point in just doing bits of the flat - it won't help to get the rent up if bits of the flat look "new" while other bits look "old".
I'll make a decision between the "rent as is" and "complete refurbishment" choices once I have the numbers to work with.