Your friends are right, although there is not really a 'right' or 'wrong' time to have a bonfire as such, it all depends on circumstances etc. The principal piece of legislation that deals with 'statutory nuisances', which a bonfire could be, is the Environment Protection Act 1990. An occasional bonfire that is not causing undue nuisance to your neighbours or immediate environment (e.g. not causing smoke to billow on a nearby highway - an offence under the Highways (Amendment) Act 1986) is highly unlikely to be a problem. But the neighbours of someone who has regular bonfires, for instance, which interferes with their enjoyment of their property or their well being etc could request that the Council serves an abatement notice, which can be followed up in the Magistrates.