St Budeaux: 11,267
I can think of one that slots in at #4 on this list - Winnersh (7,939). Possibly the smallest settlement in the country to have more than one electrified station, although I am more than happy to be corrected on this. Remember also that Winnersh Triangle is relatively recent!
6548 for Denham, with its lovely Golf Club
This as always comes down to the definition of population of an area. Tyndrum is postally FK20 and therefore comes under Crianlarich, the inclusion of which probably more than doubles the population as well as adding a third station.
Lelant and Lelant Saltings?
Population 761 in 2001
Doesn't beat Tyndrum (167) but I should think easily beats Kiveton.
Just realised the thread said 'mainline' stations, but who defines a 'mainline'? In that case what about Newark Castle and Northgate (25 376)?
I did say 'mainline' in regards to it being a national rail station as opposed to metro. tramway, light rail. I should have been a bit more specific initially.