Thanks everybody for all the tips. I will probably upgrade from what I have.
The problem with the current Tourney derelleur is that the upper pulley support was bent after an accident I had months ago. I made a tool to straighten the derailure hangar and it has done a good job. The problem now is that because it has been bent next to the upper pulley any pressure I put on it tends to pop the chain off. It's just harder to align now that's all.

As somebody has mentioned above, Tourney is ok but they bend easily. They probably flex a little bit too.
That's why I was thinking of upgrading
It won't cost that much more.

I am still not sure what I am looking for because there is a wide range of Derailleurs out there. It is a bolt fitted mechanism. Obviously Shimano. My bike is 21 speed.

I gather it comes down to the fitting and the cable fixtures as well. Maybe the size of the cog is also important
​​​​ The current Derailleur I have has a spec of 34T which I assume is a measure of the size of the lower cog?
Maybe somebody can clarify that for me.



​​