Please read my link.
You CAN'T be sent to prison for common assault unless the offence is classed as being at the very top end of Category 1. While previous convictions can push sentencing slightly higher (e.g. from a medium level community order up to a high level one) they CAN'T affect the actual category which an offence falls into.
It wouldn't matter if this was your ten thousandth conviction for common assault (and you'd also got loads of convictions for armed robbery, drug-dealing and heaven knows what else), as long as it doesn't get classed as being at the very top end of Category 1 (which, from what you've told us, would seem to be impossible) the magistrates CAN'T send you to prison.