Hi Georgina. I'm white but I work with indigenous people from the Amazon and I have friends of all nationalities and races. I'm not going to comment on the writing because others have done that very adequately, but I would totally encourage you to use your writing to explore racism, both overt and subconscious. Replacing your social worker of colour with a white person would be completely wrong. I think your exposure of the subconscious racism which was almost universal in rural England at that time, even amongst liberal-minded and consciously anti-racist people, just because of the environment they grew up in, is very accurate. Certainly you could do it with a little more finesse in the writing, having read the comments of others here. The more you can do it in a way which is just part of the story rather than sounding like an 'issue' that you want to include for ideological reasons the better.
Non-white friends criticise writers for adopting a colour-blind approach; they want racism to be discussed, to be exposed, because that is the way it will eventually be consigned to history.
Do find some people of colour to beta read for you, and listen carefully to them.
Including racist characters in your writing is not racist. It is what you do with them that matters.