ESA Assesment

Had been given an appointment over 3 weeks ago for an assesment at Luton Capability Green. I arrived there before 9am to be told that they had tried calling me yesterday, but couldn't get hold of me. I was in all day but do not answer the land line as cannot get there quickly enough. I checked the call and no message was left. I told the reception this and they said we don't leave a message as we don't know who will get it. I said that's ridiculous I live alone anyway, but you could have just asked me to call you back. They said they didn't have my mobile no. There were two other people waiting and one of them said there appointment should have been at 9.30am but were told to go in earlier. Anyway I said I couldnt wait any more than 30 mins so appointment would need to be re-scheduled.
I said I could only attend at the same place as the other in Alma street is on 6th floor, without any parking and I couldn't get down the stairs in an emergency. I was not asked to claim travel expenses. Anyway who should I complain to about this?