Client won't let me go

[NotAllThere]

... just had someone on the phone explaining that my contract may not even be valid on the basis that was only renewed verbally ...

That's somewhat testicular, since you have been working so a contract does exist, and a verbal contract is as binding as a written one, even if harder to prove. No-one works without expecting some recompense. The terms of your previous contract are likely to still apply.

[SueEllen]

I put my last invoice in at the end of last month, along with my filled out timesheet (these don't get signed). Is it really likely that they will not pay me for a month's worth of work that I completed and they charged the end customer for?

If they don't pay you, then you chase them up for it.

Use your right of substitution (post on this board) or an agency to find a replacement if there is any more work.

[The Lone Gunman]

The notice period is a bit of a nonsense whatever happens.
They wont pay you if you dont attend. Nothing in the notice period (I assume) says you have to attend the 4 weeks.

It depends how much you want to upset them.

Give them the required notice then inform them you are taking 4 weeks leave. They have already refused a sub so it is not your fault if they are left without capability.

You have a business to run. You signed a contract to work for x days. They are failing to provide that work. You can not hang around forever waiting for them to find more work.

[expat]

A few weeks ago my boss asked me if I would mind going on holiday for a week while they looked for more work for me. As a favour I agreed. ...
he's told me he is waiting to hear back on some work they want me to do, and they wont know until next week. I did mention to him that the work might not come about and was I expected just to sit at home unpaid all that time and he "understands my concern".

This is your call, but I not that if you neither walk nor give notice, you are waiting around unpaid for as long as it takes for him to find work for you. I bet he likes that! Non-mutual obligation: you are obliged to wait on his pleasure, he is not obliged to pay you. Can I have a plumber like that please? One who takes no other work, so he is always ready if I call, but doesn't charge me for that.