Thank you all for your answers. It's helped clarify a few things for me. The being walked into class by the teacher, and being walked down by the secretary is all perfectly normal. That's good to know.
But not allowing any parent volunteers is unusual, and not getting a reply to a letter is as well (I thought a simple acknowledgment would be a matter of courtesy after a letter. Hmm)
I think my problem is that I had assumed that certain things would happen or be encouraged, and then when they didn't it felt like we, as parents, weren't being involved. As a Reception teacher before I had ds (I know, I know - a school's worse nightmare ), and in various schools I've worked in, we've always welcomed parents into the classroom (to see what's been going on, to chat before and after school, as parent helpers etc) and the school's I've worked in have always had a very open door policy on involving parents, with kids being picked up from the classroom door and loads of parents helping out.
My ds's school was a bit of a culture shock, as it's not like that in reality, although they 'talk the talk' when you are prospective parents iyswim. I offered to help (not in Reception, but anywhere else) to be told they don't allow parents to help. And parents are actively discouraged from entering the school at any time. Well, that's the impression I've been given. I could be wrong as it's still early days, I suppose.
I've joined the PTA, which the school does encourage, so I can help that way. Sigh.
Anyway, thank you!