Well I have actually progressed with the development and managed to successfully test the rocket motor, well almost...lol. Here is the vid
The rocket is a RATTWORKS H70 motor with the end cut off and the cap drilled out. The injector nozzle was tapped and a 1/8th NPT to 1/4" swagelok fitting inserted and a 1/4" SS liner attached. A 1/4" swagelok valve was attached to a BL351 Futaba servo and this acts as the throttle.
I installed a small firework inside the fuel grain and attached an electric match so I could ignite it with an electric match. The firework was just stuck in place with glue.
The electric match was set off using a 12V battery and allowed to burn for 5 seconds before the valve opened 10%. This allowed the firework to burn away before opening the throttle to 100%
The rocket was at full power for 5.5 seconds before the fuel grain ran out and the flame went through the side of the casing. Here is a pic of the hole it made
The thrust was very strong and blew the grass around some 40ft away from the back of the rocket. Next step will be to make some special fuel grain and a bigger housing.