I've recently not been paid 600 by a client who I ve had a few times before.. I always get the cash upfront usually but being an idiot I let it slip for him to now having him ignore me. I feel violated and seriously pissed off. Is there anything I can do

There is absolutely not a thing you can do except learn from it.

Unfortunately chalk this up to experience...
We have all done it, I had a regular when I was a few years in, totally reliable, no issues, turned up for a booking (£100) with the 'forgotten wallet' routine.
Gave him the leeway as he had always been okay, never saw him again.
£100 at the time was no small potatoes but it was the last time so...

Keep on keeping on x

You have all my sympathy and understanding... for what's worth!
In my case what happened to you is almost impossible, but I must admit that, by being extremely selective and careful, my business volume isn't very large!
So, really, while you surely must have learnt the lesson, don't worry too much.. perhaps your mishap wasn't so despicable, to give plausile trust is a generous attitude, not something one should be ashamed of, right?

Why does low volume make it impossible for someone to leave unpaid?

I'm low volume have had new clients turn up without full amount, or having lost the envelope they were sent to cash point immediately. One had had 2 webcam bookings so knew what I looked like, knew what I may be like to get along with, despite being a professional (v concerned about discretion) used to working to rules he managed to not bring the correct fee. Others have had it but the note was still in their wallet or trouser pocket. Also miscounted whilst clients are chatted to me, they of course are saying it's correct but encouraging me to check, there's music playing, they are keen to start, miscounted once twice only realised when I checked after client has left. I do inform the person explaining I hope they don't mind me checking carefully next time. One regular I did need to come back with the difference because he is already on a much, lower rate, and always overruns even if just checking. I felt double whammied, it took more time for him to return, me to open the door, accept the difference when I needed to get on for a different commitment but it really was not fair to not sort it out there and then. This happened twice (and he did find the missing £20 each time). He now pays at the start as do 90% of my clients.

These have been a mix of new, long term regulars and repeated clients. All would swear blind they'd checked everything, counted the amount. If I have seen someone many, many times, they've spent a lot, and or are full rate I may take the chance but in general I prefer the money upfront. A few pay by bank transfer in advance which is very much appreciated.

Why does low volume make it impossible for someone to leave unpaid?

Well, in my case the two aspects are related, because, having few bookings and only for longer meetings, my policy of only accepting full upfront payments is easily applied!