DLA to PIP Debacle

I have had DLA higher rate for mobility and care for about 30 years. When I applied to transfer to PIP I waited and waited for a letter telling me of an assessment date but none came. I phoned several times to be told nothing could be done, just wait for the letter with the appointment. A lady phoned me one day saying she was just around the corner and was coming to assess me in about 10 minutes. I told her I was not informed of this assessment, I was in bed, my carer had not turned up, I needed to wash and take medication which had to be supervised because I forget what I've taken, and I was unable to get downstairs to let her in. I asked if I had to have a home assessment without notification like this as I felt I needed support during the assessment. She said you've had a letter,- I denied this strongly - and she said she would arrange for a letter to be sent to me, which I assumed, stupidly, would be another appointment for a home assessment. I was then refused PIP and all my benefits were stopped, which meant my daughter lost her carer's allowance etc, and I have to return my car which I depend on in September. The assessment was apparently due for 2nd May, but their letter telling me about it didn't arrive until 24th May. The letter was dated 13th April. How could I possibly attend an assessment I didnt know about? I am devastated. I only lost my husband a few months ago and he died thinking I would be looked after. I am now waiting for a tribunal date, but nobody will tell me when that will be! I'm terrified the same thing will happen again - no letter telling me when the assessment is, and I dont attend because I dont know about it, so then I have no right to another tribunal.

Can anyone advise me on this. Are they allowed to remove benefits when I didn't know about the assessment? Are they likely to do the same thing to me again with the tribunal? The most important question - how will the tribunal view this situation. Have they broken the law? I cannot prove the letter arrived on 24th May as there is no postmark on the envelope.
Sheila