What did you put the value of your car as? Mine was renewed at 19k which was roughly it's market value at the time -if you put down the full value there's one possible reason.

No idea how old you are, are under 25? It's possible they see that as more of a risk? There could be loads of reasons, the smallest things seem to change insurance policies! That said, I thought Greenlight always charged £50?

Definitely worth it though!