Installed Showermate 1.8-no hot water!

Hi everyone wonder if you can help?
Our Showermate 1.4 was leaking & their pump assist team said due to age it was not worth repairing.

Bought a new one off the internet...in an attempt to save money...... a friend 'in the know' said it would be easy to fit as the new one (Showermate 1.8) is the same fittings so you literally unscrew & do up the four pipe fittings on the top and fit the plug. So we literally replaced one with the other and touched nothing else.

All seemed simple and relatively straightforward......until we switched it on...
cold water all fine & shower & pump both running
turn to hot water & no water comes out at all..

It seemed so simple. Any ideas? Thanks

hot's airlocked.

Thanks....how can I sort it out?

I undid the pipe screws & pipes, heard a rush of air escape from the hot side...did up again & still the same happened. How can I get rid of the air lock?
Thanks
Sorry am a novice at this so help appreciated