Your barrister is probably right in indicating that "it's too close to call".
Unlike offences prosecuted under assault charges (e.g. 'ABH' or 'GBH') there are no written guidelines for judges to refer to when sentencing. So, unlike, such cases, we can't simply take a look at what the judge has to read and try to predict how he might think.
However the fact that you refer to a barrister, rather than a solicitor, suggests that the case has been referred to the Crown Court which, in turn, suggests that the magistrates thought that their sentencing powers (of up to 6 months imprisonment) might not be sufficient to deal with the matter. That, though, should not be taken as an indicator that a prison sentence is inevitable, as judges often see things very differently to magistrates.
Despite the absence of formal guidelines, however, judges are required to consider precedents set by courts in the past. In R v Fox and Hicks, in 2006, the judge was required to consider sentencing in respect of affray convictions where there was no direct ('one-on-one') violence between opposing football fans but where debris had been thrown and threats made. Hicks (who was 25, and with previous convictions) was sentenced to 12 months imprisonment. Fox (who was 35, a family man and with no previous convictions) was shown to have participated to a lesser extent but he was still sentenced to 8 months in prison.
The judge will almost certainly pass a custodial sentence (probably of around 8 months, considering the similarity of your offence to that of Hicks - although possibly rather longer because of the 'one-on-one' attack). He will then have to consider whether that sentence can be suspended. If I was a betting man I'd probably put my money on him choosing to do so (simply because I've observed lots of Crown Court cases and know that most judges do try to keep people of prison wherever possible) but I'd still advise packing a bag before you attend court because, as your barrister says, it's '50/50' really.