Not opened bonnet for a while and today wanted to open it to refill screenwash. I pulled the catch at the side of the front passenger footwell but nothing happened. The catch does seem a bit loose/ flimsy so I don't want to force it but I couldn't feel or hear anything happening and the bonnet doesn't seem to have moved.

you need to check if the plastic handle is actually moving the cable or has come detached from it. I have had many cars where the handle has broken and pliers needed to pull the cable (and they were probably all caused by lack of maintenance (oil/grease) on the mechanism under the bonnet)

Also sometimes when the bonnet catch area gets corroded, it can help if someone pushes down on the front of the bonnet, you then pull the lever and hold it out, and then the other person lets go of the bonnet and see if it pops up. Worth a try.   If the cable has come detached from the mechanism at the front end it will be a bit tricky.