I stayed at Helena's place for 8 nights and everything was perfect.
Helena welcomed me the 1st day and gave me all the instructions for the house (heating, etc.) and spent time with me to let me know about all services in Notting Hill.
The apartment is perfectly located: at 3 min walking distance from Portobello Road - plenty of nice lively trendy cafes & restaurants, and the famous open-air market - and at 10 min to the underground station of Notting Hill for Central Line (red one) which crosses all London and brings you easily everywhere directly or through connection with other lines.
The apartment is confortable, clean, well decorated, the kitchen is small but functional, the bathroom new and clean. Although in a basement, it is luminous enough and silent. The kitchen gives access to a small garden, which you can use to have breakfast / lunch, weather permitting.
I believe Helena's place is the perfect choice for a single person or a couple and would definitely strongly recommend it for the reasons above and for the very high quality / price ratio.