He is part of the Leading Mathematics Teachers scheme, which is proving successful nationally in improving the way the numeracy strategy is tackled in schools. On these days, three or four teachers arrive at Yerbury primary, in the London borough of Islington, and Mr Blackburn discusses his lesson with them before they watch him in action.

Today his maths lesson starts with a card game with children having to guess whether the next card will have a higher or lower number than the previous one. They all join in a number-crunching session, and then out come the whiteboards for more work on the concepts of higher and lower. “Write an even number greater than 50 but less than 100 and check with the person next to you,” he says. The lesson moves on to the topic for the day: shapes. Mr Blackburn dons a helmet and the class has to tell him enough about the shape on the ront for him to guess whether it’s a square, hexagon or rectangle .

After 30 minutes on the carpet, the children move to their groups, arranged according to ability. One set has to write the properties of a range of shapes; another has to create symmetrical changes. For the final five minutes, it is back to the carpet for blindfold noughts and crosses.

Leading teachers get no extra money, but it does bring professional recognition. “I believe in open doors and the children like it,” he says. According to the Office for Standards in Education, schools could make better use of the scheme. It found that the teacher-observers were not always sufficiently briefed about what to look for.

Mr Blackburn said: “It’s important observers understand the context, and I will explain what I am attempting to achieve.” Nominated to be a leading teacher by the head at his previous school, he makes no claim to be a natural mathematician: “When I was at school maths was like a secret society. You could either do it or you couldn’t and often I couldn’t,” he says.