Mine is really loose, guessing the cable has slackened over the years

I could replace but that seems like an awkward job, is there adjustment on the cable I could do instead?

Mine was loose like that on the switch inside, and also the bonnet wouldn’t lock when closing it.

A garage thought I would need a new cable but it's been working perfectly since did this. 

First I used a little bit penetrating spray (careful not to use one which degrades rubber as hoses etc underneath) but what really seemed to help was a few drops of lubricating oil I had for my bicycle chain, over the spring and where you see levers sliding against metal in the locking assembly bonnet itself, where the cable connects to the bonnet lock, and work it into the assembly by moving the lever in the bonnet back and fore.

Where the cable connects into the spring assembly you can also push the cable back a bit if it’s failed to go back itself which got my bonnet to lock when it didn't.

After that make sure it is kept lubricated as part of a servicing schedule, white grease is usually what they use at mine.