How do you tighten up the lock ring?
It’s just a cassette lockring tool. Halfords sell them if you need something local, or CRC do one if you’re going mail order.
…or just see if your LBS has one.
You will need a wrench to tighten it, tho.
Should I be using a special tool?
Yes. Although it’s not really special.
At the moment I have just used my fingers to get it as tight as I can
I would strongly recommend popping into your LBS, and get them to nip it up for you, while you source a tool.
I have noticed some very, very slight movement in the front wheel when front brake is applied and I rock bike back and forward. Could this be the reason for the lock ring working loose? Thanks for any help.
Hard to say while you have a loose lockring. Get that tightened up, and then see if you still have any movement.