anyway, so do you think that a mental illness was a significant influence on his erratic behaviour?

Certainly. Especially after reading that he couldn't understand why people were still interested in him, couldn't understand what he had done to earn the ongoing affection. That tells me he had to be a bit crazy. Or ultra modest.

but what would that ahve to do with a mental illness?

I read this thing on a website titled "preventing schizophrenia"

Research conducted in the past decade indicates that schizophrenia is due to a genetic predisposition and environmental stressors early in a child's development (during pregnancy and birth, and/or early childhood) which lead to subtle alterations in the brain that make a person susceptible to developing schizophrenia. Additional environmental factors and stresses later in life (during childhood, adolescence and young adulthood) can either damage the already vulnerable brain further and trigger schizophrenia or lessen the expression of neurodevelopmental defects and decrease the risk of schizophrenia. In fact experts now say that schizophrenia (and all other mental illness) is caused by a combination of biological, psychological and social factors, and this understanding of mental illness is called the bio-psycho-social model.

It has been speculated that he suffered from Schizophrenia; Roger Waters actually said that he did in an interview. But, there are also theories of Aspergers Syndrome and dementia. Obviously LSD use does nothing to help mental illness, so that definitely was a factor. I honestly think he was a genius. Just nobody understood him.
~Shine on you crazy diamond~