> What was the fusion in chicken tikka masala?

It has tomatoes in it for one.

> And come to think of it, what other new dish has the British Indian restaurant industry come up with in the 50-60 years after the chicken tikka masala.

Chutney (the type consumed in UK), for example: https://en.wikipedia.org/wiki/Anglo-Indian_cuisine

But 50-60 years is not a long time. Things do move a lot quicker now that food import routes have made it possible to get most exotic components to most elsewhere places without growing them on-site. But take a step back on what you look at as quintessentially "local" food anywhere and there's always movement. Nations form, crumble and split, peoples migrate, etc.