Logging back on to say definitively that you can get to Brandon with split ticketed off peak or anytime returns, don't pay £45.
Absolute cheapest is LST-CBG, CBG-ELY, ELY-BND. Change at Cambridge or Cambridge North. £24.80
2nd and slightly easier is LST-CBG, CBG-BND, and you can just change at Cambridge. £27.20
Both of those you're restricted to Greater Anglia trains.
Fastest is KGX-ELY, ELY-BND. £31.20
I believe you can get on at Tottenham Hale with a ticket from LST becuase breaking your journey is allowed, but I'm less clear on that.
Logging back on to say definitively that you can get to Brandon with split ticketed off peak or anytime returns, don't pay £45.
Absolute cheapest is LST-CBG, CBG-ELY, ELY-BND. Change at Cambridge or Cambridge North. £24.80
2nd and slightly easier is LST-CBG, CBG-BND, and you can just change at Cambridge. £27.20
Both of those you're restricted to Greater Anglia trains.
Fastest is KGX-ELY, ELY-BND. £31.20
I believe you can get on at Tottenham Hale with a ticket from LST becuase breaking your journey is allowed, but I'm less clear on that.