Flexible over arrival time, very clean and well presented (except mildew around the bath rim).

Downsides:
No locks on doors
No water glasses in rooms - got chided for requesting one!
Sunday "breakfast" (toast) finished by 9am. A loud knock on the door at 0830 woke us to remind us of this, whether we wanted it or not.
Further knocking at regular intervals to remind about the 10am checkout.
Fighting the other residents for the single shared bathroom made achieving this deadline difficult.

All but the most hardened travellers (and early risers!) may want to look elsewhere.

Very friendly and considerate reception. Superbly placed for convenient access to underground, buses and shops.

Comfortable accommodation, which was enhanced by the absence of hum from TVs, making for a peaceful and less stressful visit than anticipated. No tea and coffee making facilities in the rooms.

Northfields B&B is a tastefully furnished, clean and comfortable place. The landlady Deirdre is very friendly and offers help whereever it might be needed. She gave me a very warm welcome indeed. I especially liked the fruit salad for breakfast that she made fresh every morning.
There is a very good bus service a 5-minutes-walk away that will take you to a choice of two Tube stations in a very short time. The journey into Central London takes around 45 minutes.
There is only one thing that one must know before booking. The single room is very small and only good for those who do not mind living directly out of a suitcase for a couple of nights.

Northfields BnB is located in a very nice, peaceful neighborhood and additionally is in a strategically good location (ca 20 min to the city centre by tube and approx same time to the airport).

Further, the rooms are cosy coupled with really nice service offered by the landlord. The breakfast includes various choices (e.g. cereal, toast, ham, cheese and even porridge etc.)

In regards to the room rates, it is definitely reasonably priced, especially taking into account the average rate in London. In regards to my personal experience I have paid from 45-50 GBP per night depending on the duration of my stay.

Hence, it does not matter whether you are visiting London for business or just for fun and pleasure, Northfields BnB is a good choice in either case.