Just to clear things up OP, you stated this: "Cue fairly large trolley bloke who has SN of some sort (not sure what they are because he never talks to anyone as far as I know)" which, without calling you a nasty "SN basher" (wonderful term 
) is ignorant. Does everyone with special needs have to wear a banner proclaiming which special needs they suffer from?
Also how come you then say that he is normally polite and helpful when in your first post you say he never talks to anyone?
And I'm sorry but did I miss where she said he had been there 10 years?
onefatcat: "Supermarkets often employ people with SN to collect trolleys etc so she is probably right to assume he has SN," what a presumptious pile of poo. Just wanted to point that out to you, I don't think that ridiculous statement needs replying to.
We are not talking about a bloke with a hoodie on, we are talking about a member of Waitrose staff. One whom the OP is presumably familiar with although she has never heard him speak and then she has (is she confused?) and one which she knows has special needs.
He did not come out of a street corner shouting at her. He saw her go the wrong way around the car park, he pointed to the ground and shouted "One way". She doesn't answer so he goes closer and says loudly "One way" and then continues saying this as she walks to the store.
In her OP she did not specify that she had said sorry, she merely stated that she'd told him she didn't think it a problem.
How, may I ask, is this harrassment or intimidating?
I'm sorry to have to break this to you, but people with special needs sometimes tend to talk loudly. They don't have to be deaf to do so, they just do. They are also sticklers for the rules and get upset if they see the rules being broken.
So how about a little compassion and understanding for someone who is paying his taxes, working and trying to live a normal a life as possible?
Yes sometimes you do have to make exceptions for adults who have special needs, it's hardy going out of your way to do so though is it? Talk to them like you would do anyone else and show them the same levels of respect and you'll be respected in turn.
I wonder what response she would have given if an ordinary shop member had pointed out that she had driven the wrong way?
I'm very sorry that you found him abrupt, but this is the reality of adults who have difficulty communicating and if you know him to be polite and you know him to have special needs then you make concessions - of course you do! Because you realise the difficulty of the situation and you realise that you were in the wrong. You don't make a complaint against him because in reality he will now be taken away from the public and placed in the storeroom.
The public obviously aren't ready to accept a grown man who talks louder than he should, who cannot articulate sentences very well and who is a stickler for the rules.