MIRA 415 COLD water even after replacing cartridge?

Hi Guys,
After alot of frustration I have no choice but to turn to you and ask a question.
I have a white MIRA 415 (Black Cartridge) shower which is approx 15 years old. It started off with going cold from hot and when you turn it off and on again it would be nice and hot but then go cold again.
Then after a few days it started to leak from the head.
So upon investigating I found ordered and changed the cartridge.

Now when I turn it on it gives me slightly warm and cold but no HOT.

I wanted to know the following:
1) is my showere completely bust?
2) Will I need to get a plumber out?
3) Is it easy to fit a like to like Mira shower?

Sar
January 2013