Back in the heady days of September 2007 I was at an arms fair in the Docklands in London, though to avoid confusion I should add I was outside speaking against it, rather than inside browsing. The crowd I addressed was a mixture of Quakers and crusties and for my troubles the police stopped and searched me as I left the event, an incident that I wrote about in Cif.

When the police conduct a stop and search they have to fill out a form giving their reasons and hand a copy over to you. On mine they wrote that Mr Thomas appeared to be an "influential individual" – a quote I intend to use in future publicity – and had attempted to walk past the police with an "over-confident manner" – always a sure sign of criminal intent. Maybe I am wrong, perhaps there is a forensic linkage with having an "over-confident manner" and criminality, perhaps the police routinely chase suspects through our metropolis shouting, "Stop him, he's got a jaunty demeanour!" But I got the distinct impression the police were stopping me because they thought they could.

My rakish over-confidence might be the reason I was stopped but the official purpose was to look for items I might use to commit criminal damage (the arms fair had been subject to a paint attack earlier in the day). So exactly what tools did the police hope to find rummaging through my wallet? Unless my wallet possessed some Tardis-like qualities it was unlikely that a large crowbar might clatter out from between a photo of my daughter and my British Library card? Wasn't this intrusive as well as unlawful?

Although protesters are often targeted for stop and search, often claiming these are unlawful, they seldom seem to put in official complaints. So with the help of solicitors at Fisher Meredith I brought a complaint against the police. Being Britain the first step in a complaint against an official body is for the very body you are complaining about to investigate itself. And lo the police did find themselves innocent.

In official interviews the officers who conducted the stop and search described me as "pleasant and conversational throughout the incident" and thus were "surprised and disappointed that Mr Thomas has made this complaint". Please note that it is me that has disappointed them in this complaint, we had a bond in those moments you see, a brief passing moment of pleasant intimacy, then I let them down. I don't return their calls, ignore them in the street and am later seen being searched by other police officers. In their logic my decent behaviour exonerates their bad behaviour.

So had I been rude or surly would this have implied guilt on their part? If when stopped I had responded by saying, "Fuck off copper", would they have blinked and spluttered, "Oh blimey, you got me bang to rights guv'nor ! It's an unlawful search, an' no mistake." How else are we meant to respond to the police if not politely? If innocent people respond rudely they become guilty by default. Suffice to say the police deemed there was "no case to answer".

So pressing on, the issue was put before the Independent Police Complaints Commission (IPCC). Now the IPCC generally attack the police with all the effective ferocity of a moth taking on a lightbulb. So I was somewhat taken aback when at the end of last year they declared that they "consider[ed] the stop and search of Mr Thomas and the subsequent search of his wallet was unlawful". And that "it would appear that the officers had misinterpreted their powers under PACE (Police And Criminal Evidence Act)". In an earlier point in the investigation the police said they had stopped everyone at the demo with a bag, this the IPCC said was "an indication that the officers did not consider whether or not they had reasonable grounds to suspect each individual may have been in possession of items to use in criminal damage".

Fifteen months after the event the police have been found to be acting unlawfully. So what sanctions do the police face? "The appropriate way to address the failure in standards is that the officer is given words of advice," says the IPCC, "Such advice is neither given nor received lightly and is delivered by a senior member of the officer's management team." I am sure you will agree this is a daunting sanction.

The debate about the use of stop and search – be it protesters or young black and Asian men, be it in the case of stopping knife crime or deterring terrorism – is one that has understandably perhaps been fixed on the police results rather than the times they get it wrong. But it is in the cases where they get it wrong that attitudes towards police are sharpened and the rights we feel we have as citizens practically defined. So I am writing to the police requesting a formal admission of liability on the part of the commissioner and damages for assault and false imprisonment.

The Convention on Modern Liberty will begin in London on Saturday 28 February at 9.45am at the Institute of Education, 20 Bedford Way London WC1. Other sessions, with live screenings from London, will take place at Trinity Centre, Trinity Rd, Bristol; Student Council Chamber, Oxford Road, Manchester University; Cambridge Union, Bridge Street, Cambridge; Institute of Advanced Studies, University of Strathclyde, Montrose Street, Glasgow; Peter Froggatt Centre, Queen's University, Belfast. The venue in Cardiff is yet to be confirmed.

For information and to buy tickets at £35 (concession £20), please visit: modernliberty.net