Police said that at Lewes Crown Court, on February 16, Peter Gillett, 59, of Arundel Road, was convicted of seven counts of non-recent sexual offences in Crawley, some involving multiple occasions, against two young girls and a young boy.
Sentencing awaited the outcome of a separate trial for Gillett for possession of a stun gun found at his address when he was arrested in 2016, police said.
They added he was convicted of that offence, too, and was given a 12-month sentence to run concurrently with the sex offence sentencing.
A court-imposed reporting restriction had prohibited publication of news of the sex offence sentences until the stun gun offence had been dealt with.
Police said Gillett will be a registered sex offender for life.
He was convicted of: two counts of rape, two of indecent assault, one of multiple offences of rape and indecent assault, one of causing actual bodily harm, against a girl; one count of gross indecency and a count of child cruelty against a boy and one count of indecent assault against a girl.
He was found not guilty of a count of rape and a count of indecent assault against the first girl and one count of indecent assault against the boy, police added.
The prosecution followed an investigation by detectives from the West Sussex Safeguarding Investigations Unit.
Detective Constable Rees Hopcraft said the offences came to light in 2016 when the two female victims, then adults, contacted police ‘after learning of online postings suggesting that Gillett had committed sexual offences against the boy’.
DC Hopcraft added: “We investigated and gradually uncovered a series of offences of sexual abuse by him against all three victims when they were young and vulnerable children, over a period of years.
“All three gave evidence against him and after a long trial, during which Gillett discharged his counsel and defended himself, the jury found him guilty of the sexual offences.
“We will always take seriously and follow up such reports, regardless of how long ago the events are said to have occurred.”
Gillett filmed giving a speech at the EDL demo in Coventry 2016.