Acorn superglide stairlift stopped ?

I've got a really strange problem with an acorn 120 stairlift. The bateries have just about failed so we fitted new ones only to get an F fault indicated, this has been translated to mean a motor fault but after fully testing the motor its fine.
we put the old batteries back on which only read 11.8v & it works all be it very slow .. we tried a different set of new batteries.. F fault again.
everything else checks out ok. we've put the old batteries back on for now but it barely makes it up.. could it be the pcb? please help.