stannah 260 stairlift problem?

Hello,
I have a stannah 260 stairlift that is no longer required.
The stairlift hasn't been used since 2011 apart from a couple of times a few months ago.
Yesterday, I tried the chair and it will only travel about 12" then stops and bleeps. It will not go up or down until I switch the power button off and on again.The strange thing is that the stairlift works fine when nobody's sitting on it.
Could you please suggest any steps I could take to sort the problem out.I am reluctant to pay for a home visit as I want to try and sell the chair.