Is it possible to get a proper diagnosis if you don't have anyone who can speak about your behaviour as a child?

Yes it is!  There are things you can do.

Have you any school reports?  They often show traits you had as a youngster.  Remarks like 'Does not listen', 'Clumsy',  ' Not communicative', 'Fidgets' 'Does not pay attention in class', 'talkative', 'Intelligent but under performs', 'Slow to pick up', 'Does not understand what is required', as well as inconsistency within a subject or over a range of subjects, and different performance between written and spoken work.

Even if you haven't got the reports, can you remember what was said.  I wrote a potted history which went on forseveral thousand words, about my life, both from the aspect of myself and what I had been told.  I included difficulties I had in finding work, problems in interviews (I couldn't work out what was being asked) and difficulties you have with sensitivities such as not liking wide range of foods because of strong smell, appearance, or taste.  Have you been pulled up for staring, not making eye contact, or fidgeting when you didn't realise you were doing it?  Anything in fact that you think might help.

Print it out and send a copy as well as keeping one yourself for the assessment.  And don't be afraid to refer to it during your assessment.  

I was in my sixties when diagnosed so the whole process relied on what I said and my answers and reactions to questions.  The more information you can give, the easier it will be for the psychologist/psychiatrist to reach a conclusion.  And there is something that makes you think you are autistic, and if this proves not to be the case, the assessor should be able to get an idea of what the problem may be.

Yes it is!  There are things you can do.

Have you any school reports?  They often show traits you had as a youngster.  Remarks like 'Does not listen', 'Clumsy',  ' Not communicative', 'Fidgets' 'Does not pay attention in class', 'talkative', 'Intelligent but under performs', 'Slow to pick up', 'Does not understand what is required', as well as inconsistency within a subject or over a range of subjects, and different performance between written and spoken work.

Even if you haven't got the reports, can you remember what was said.  I wrote a potted history which went on forseveral thousand words, about my life, both from the aspect of myself and what I had been told.  I included difficulties I had in finding work, problems in interviews (I couldn't work out what was being asked) and difficulties you have with sensitivities such as not liking wide range of foods because of strong smell, appearance, or taste.  Have you been pulled up for staring, not making eye contact, or fidgeting when you didn't realise you were doing it?  Anything in fact that you think might help.

Print it out and send a copy as well as keeping one yourself for the assessment.  And don't be afraid to refer to it during your assessment.  

I was in my sixties when diagnosed so the whole process relied on what I said and my answers and reactions to questions.  The more information you can give, the easier it will be for the psychologist/psychiatrist to reach a conclusion.  And there is something that makes you think you are autistic, and if this proves not to be the case, the assessor should be able to get an idea of what the problem may be.