Ian Smith's photo of Cobham's SMS at ChislehurstThe SMS

AEC Swift standee one-person operated bus
In the 1960s, London Transport faced a range of problems, including falling passenger numbers, increasing congestion (from the cars bought by former passengers), staff problems and a changing political climate. 
By the time the last of the Routemasters was delivered in 1968, the decision had already been made to convert wholesale from double-deck crew to single-deck one-person operation, following the fashion in the rest of the country.  The result was the Bus Reshaping Plan, which started in Wood Green and Walthamstow with major upheavals to the pattern of service and the widespread use in those areas of 'flat-fare' routes (now that all London bus journeys cost the same, it is easy to forget that crews used to have to cope with complex fare charts for each route).
Cobham's SMS follows a Leyland National while working the 227 on the Sidcup running day in 2002.
© Ian Smith 2002, used with permission
The buses used were AEC Merlins, a large-engined version of the standard AEC Swift.  But at 36 feet long, the buses simply didn't fit many of London's roads (see the admission on the notice of the 178 termination).  Facing this, and with major customer dissatisfaction, it rapidly became apparent that London Transport had made an expensive mistake.  An attempt was made to put some of this right by changing to a shorter version, the 'short Merlin' or SMS, actually a standard Swift.  But this too was to prove a mistake - the smaller engines (or perhaps the maintenance regime in place) couldn't stand up to the rigours of London bus work and the buses failed frequently.
All too soon, double-deck buses were back in fashion and the inability to put the Merlins and Swifts through a standard overhaul procedure led to their early withdrawal in London.  Most Merlins had gone by 1973, only five years after their introduction; some of the Swifts lasted until 1981.  Compared with their predecessors, these buses saw a very short period in service; relatively few survive.
An example of this important but unhappy period of London Transport's history is preserved at the Cobham Bus Museum, who maintain SMS369 in working condition.  This bus will be in service on Easter Sunday on routes S2 (the former 208) and 236, both of which saw the class in service in the 1970s.