Re: There, there children. Sorry your pacifier is b0rken.
For the routes the OH works on, there are two types of timekeeping employed - reporting points and headways.
On routes using reporting points, there are a handful of places along the route (typically the start and end points plus a few selected stops at regular intervals) at which the bus is expected to turn up at a specific time. At other points along the route, *including any bus stops not designated as reporting points*, the bus is free to arrive/depart at any time as required in order to ensure it arrives at the next reporting point as close to the required time as possible.
On headway routes, buses are instead expected to maintain a fixed time offset to the bus ahead of them, so that whilst the exact time of arrival at any given stop isn't defined, the interval between buses at each stop ought to be consistent.
Bus operating companies take this quite seriously, and drivers can and do get hauled up in front of their line management if they're regularly seen to be ignoring the reporting point or headway timings without a damn good reason (e.g. emergency roadworks throwing the whole schedule into disarray). The amount of monitoring of London bus and driver performance that goes on is quite something to see if you're still of the belief that buses are these antiquated means of getting from A to B, little more than giant tin boxes on wheels still pottering around in blissful ignorance of the march of technology all around them - realtime position tracking to enable the onboard stop announcements and remote monitoring of timings/headways back at the control centre, onboard acceleration/braking/cornering force logging (which are also used as a way of beating up the drivers if they're seen to be driving in a manner not in keeping with whatever policy their company has - e.g. driving for maximum passenger comfort, driving to maximise fuel economy etc), multi-camera interior and exterior CCTV setups, system diagnostics/fault logging (particularly so on the hybrids/other alternative fuel buses)...
So if a London bus doesn't arrive within the time interval stated on the bus stop, it generally means something has gone wrong further up the route, and only rarely will it mean that the bus is being driven by someone who really doesn't care about maintaining correct timings. There certainly are some drivers like that, they just tend not to last very long in the job these days given how quickly their employer will come down on them like a large quantity of rectangular fired clay building blocks - most drivers are only too keen to avoid the risk of disciplinary action, so do attempt to maintain the required timings if possible.