I have two Dapol Brits including one of the new batch. Both go round second radius curves without any problem so 15" is certainly OK.
I used to see the same Fish trains when they passed through Whetstone, just south of Leicester. Brits 70035-70041 inclusive were allocated to Immingham in the early 60s & all were used on the Grimsby-Whitland fish trains which passed through Whetstone approx 9pm every evening.
Hope this helps
David