Thanks Rick for the article, it reminded me I forgot to put tyrosine in the sequence. I did my first post from memory at 6-00am!!
The full sequence is.
protein---->phenylalanine---->tyrosine---->levodopa---->dopamine

Phenylalanine, tyrosine and levodopa are all amino acids, but differ by the number of hydroxyl groups on the aromatic ring.
I originally tried phenylalanine since it is readily available in UK health shops. I did not find tyrosine. Surely someone somewhere has tried this before, with either phenylalanine or tyrosine, even on animal testing.
Ron