You shoudn't need to buy the entire assembly because the pump/filter assemblies are seperate from the accumulator.on the later cars such as the 560 there are two pumps and in that case you can get robbed severly .
Try going to a breaker and buying an accumulator from any of the W126 cars, Including the 280SE/SEL or even a part from a W116 450SE ( the 6.9 even has the same bit ) .In any case, get under the car and replace all thosae rubber hoses,the climate in the UK is not conducive to long life on the hoses I notice( My local breaker gets lots of mercs here for wrecking from the UK...)
http://www.hitcity.com.au/GermanWreckingCo.info
Good luck!