I recommend Cherry Bikes on the left hand side of the 118 just before the 132 when heading out of the city.
I bought a Giant MTB from it and used it for several years without looking after it. Then, after leaving the bike in the sun unused for several months, all the cables became brittle and failed. It replaced them all and fully serviced the bike so it was as good as new, all for a very reasonable price.
Look at the reviews on Google maps.