Please check carefully to ensure that the pigeon can't escape or be helped to escape. Check to see whether there are any other pigeons trapped there and whether any of them have died. Make a note about the condition of the pigeon and the immediate risk. Make certain that you will be able to give whoever you call the exact location of the pigeons at risk.
If the pigeon is not in distress then the first people to contact would be the shop, council or organisation responsible for the netting.
You will find that some say that they will have to call out the pest controllers responsible for installing the netting and that rather than releasing the bird they will kill it. Please be aware that this would be an offence under the Wildlife and Countryside Act 1980. Pest controllers work under a General Licence issued by Natural England.
The conditions of the licence requires them to consider lethal control as a final resource, so killing a pigeon that became trapped because of their poor workmanship would be a violation of the licence and would have to be reported to your local wildlife liaison officer for investigation. The person responsible for the netting should be made aware of this.