offer 10% below the asking price?

[itdc]

house on the market for about 2 months, price at the range of 900K (in london), based on the rightmove 'move improve' app, it is valued at about 750-800K. 1 house sold nearby in early 2015, 150 sqft bigger and much better conditions at 850K. and other house sold sept 2014 with new loft conversion (one bedroom more) and bigger size extension sold at 870K.

Does it make sense to start offering at more than 10% lower than the asking price (e.g. 800K ?)

Yes. Back up your offer with your comps and explain your reasoning. Emphasise your good position (if you are in a good position) and see what happens.
We haven't had any luck with this strategy (we offered 6% below because of comps and need for work) and have been turned down with no price guidance, but the vendors don't need to sell so....
Good luck!

Often the agent selling will know the position of the vendors and how serious they are about selling and what price they need to achieve. I think 10% lower is a good starting point.