Keith Tilley’s involvement as a ref only went as far as local games when his children were small (as parents know these are the most fractious matches of all) but he still thought he had more to learn.

He was one of a motley team of men and women who turned up at the Bristol Rovers memorial ground to attend a bite-size course on the offside rule.

The free courses, run by the Learning and Skills Council with local providers, have proved a huge success. Last year courses included belly dancing, sending an email and writing a CV. Around 70,000 people took part in more than18,000 courses throughout the country and many enjoyed them so much they came back for more.

Last week, instead of settling down to watch France-Uruguay in the World Cup, the group of civil servants, office workers, and a radio presenter took to the classroom and then the training pitch for their lesson.

Their teacher was Steve Dunn, a premiership referee for the last seven seasons. His highlight up to then had been refereeing last year’s FA Cup final between Arsenal and Liverpool.

“Offside is a very difficult decision to make correctly. There are always going to be errors made. You just hope they are few and far between,” he said.

He remembers ruling an Arsenal goal offside in the early 1990s, watching the video later and realising he had got it wrong. “George Graham told me at the time I had got it wrong, and he has made the same point on many occasions since.”

But this session was meant to be “fun, an easy way to experience learning”.

One of the participants on the course is Zoe Hanson, a radio presenter and “gashead” (Bristol Rovers supporter - they used to play near the gas works).

She says she thinks she understands the offside rule “well-ish. But I could not put it into words. I want to be able to hold my own in the pub during the World Cup. Normally I shout offside when the flag goes up.”

Tamsin Lambert, a contracts executive, does not play football or even watch it. “But people go on about it all the time and I do not understand what they are on about.”

Mr Dunn proceeded to try to convey in one hour what would normally take up to four hours. It is not an offence in itself merely to be in an offside position. A player is offside if he is nearer to the opponents’ goal than the ball or the second last defender. A player is offside if he is interfering with play or an opponent (laughs at that).

He waves his flag and indicates what each movement means. Then on to what is obviously a bugbear. “I wasn’t going to mention Andy Gray (the Sky Sports commentator) but I think I have to.”

Mr Gray is often critical of assistant referees who delay raising their flag and so the offside decision appears late. But, insists Mr Dunn, they cannot flag for offside until they see where the ball goes.

Then the group moves to the pitch and don their bibs. The blues are the attacking side “because they don’t do a lot of that in this ground”, says Mr Dunn.

They take up their positions as goalie, strikers, defenders, role-playing what is and what is not offside. They make a defensive wall.

“You know where you have to hold yourself, don’t you?” says one of the men.

In hockey they have got rid of the offside rule. Will this ever happen in football? “I hope not,” says Mr Dunn, “or we’d all be out of a job.”

The goalkeper is reprimanded for attempting to catch the ball. Nothing wrong with that, you think, but they are rehearsing what happens if the ball rebounds off a post.

“I spoil enough games, I don’t want to spoil this one as well,” said Mr Dunn.

The game over, everyone is enthused and the ref says he expects to see some of them on the next referees’ course.

Zoe says she would still find it difficult to put into words, but now knows when something is offside, and when it is not, which she thinks will inform her shouting on the terraces.

But a little learning is a dangerous thing. I thought I understood the offside rule. Now I am not so sure.