Thanks to all for the additional input.

So if I decide to go for a new derailleur such as the Tourney suggested by xenologer, how will this affect the chain required.
I will check the number of links in the current chain, but I have seen 114 link and 116 link chains suggested for example for the Altus derailleur, how do I determine what I need for the Tourney?

EDIT: It's OK, I found the answer on ParkTools.

Thanks again.