Some more info ...
When we found the tick, it was obvious it had been there a while, it was massive! I took the tick out, gently, and looked at it and all its legs seemed to be there.
We cleaned the bite and put anti-bacterial spray on it. We cleaned and checked it for the red ring that is the sign of limes disease a few times after that, but it seemed to be fine.
DS has very thick hair, we didn't notice the sore till the end of August. His head had a raw, sore patch, which had spread to much wider than the original bite and was yellow and flaky as well as red and scabby. The first doctor we was didn't believe it was from the tick bite (he didn't say this outright but I suspected it).
He prescribed a mild steroid cream suitable for treating skin conditions. After washing and applying the cream every day, the wider infection cleared up. The yellow flakiness had gone, and it was just down to the penny-sized circle, which wasn't budging.
The next doctor confirmed the previous doctor didn't believe it was from he tick bite, but it was clear to the new Dr it was the most likely cause. He said the previous cream had been very mild, and he has now prescribed DS "Betnovate Scalp Application". The penny-sized sore still isn't budging.
We're going back this week, but I'd like to find out what it might be beforehand. Maybe a bit of leg is stuck in there afterall, even though they looked intact at the time? (We put the tick in a glass of water so we could have a good look - yuck!)