There are a number of official reports which support our case. here are some of them:
October 2012. SYPTE Annual Footfall Survey data. Key points : Number of pedestrians passing through the station remains steady at around 3000 per day. Massive increase in rail users using the station.
Select Committee Report. Look at Section 4 - Ticket gates Not the Only Solution.
East Midlands Rail Franchise document
This is a large (170 page) pdf document, but we draw your attention to section 11.8(b) - p99, which gives EMT the option to meet the franchise obligation by providing "reasonable revenue protection measures on trains", which can mean more staff to sell and check tickets.