Hi, I also have a Stannah Model 240 stairlift.
My question added recently was has anyone replaced the 2 x 12v batteries themselves, as I'm after a step by step method on how to pull the chair apart and replace the batteries.
In relation to your chair symptoms, it sounds very much like what we observed when our batteries died 6 years ago and it has happened again now.
The expensive Stannah guy originally put 2 new batteries in and away it went until now. Also check that none of the safety switched are obstructed (triggered) as that will prevent the chair from activating.
Lance Gledhill
September 2014