What are the legal aspects (if any) of leaving my existing company if I've approached my client to go work for them on a permenant basis? Had enough of consulting lifestyle.

It depends on your contract and on the nature of your relationship with your current employer (and the nature of the relationship with the client). Commonly there will be a clause about not working for competitors or clients for 'x' amount of time (e.g. 6 months) after leaving the firm. Similarly, it's common that the client will have a clause in the contract they hold with your firm, specifying that they won't poach each other's staff during the course of an active engagement without prior agreement.In reality, it is near-impossible to enforce these clauses and all parties will be pragmatic - if you aren't very senior it is unlikely to be an issue. Needless to say, it wouldn't be a good idea to break the news or make the move if you're in the middle of a project with that client as that could cause problems in how you manage the transition and the changing nature of your relationship with the project team.

and it would be very very very serious if you were very very very very senior.

Thanks for the advice. Fortunately I'm not "very very senior". More concerned with the contractual clauses that prevent me from moving - so time for me to check out my contract... Aren't there laws re: freedom of labour to move? Those were the legal aspects I was thinking of. I heard of companies threatening to take action - but how realistic are those threats?

As per Bosman, you can go where you like