It was a few years back now but I went to see the local AME, pre the £400 blast at Gatwick, which confused him as he was used to doing renewals only. However (and he did charge a fee, but it was a lot less than wasting £400 for a failure) he went through all the tests including the EEG and ECG and tentatively told me it would be worth going for the real thing. I think it was money well spent.
The optician visit is a good start though.