How many supports will a 3.4m workspace need?

[KatyMac]

It'll be supported by a baton at either end & along the back wall - will one cupboard in the middle be enough or would I need 2 evenly spaced?

Just measured and it's 2.3 - I am a dork!

That will be fine with just a middle support

Work surface you mean? And it depends what it's made of and how thick.
1.2m unsupported front would not be acceptable for 30mm chipboard as it would sag. Max would be about a metre. For quartz it would be no issue as it's bomb proof. Solid wood again should be fine.
With suitable clearance you could get a hidden rail under the front though

Possibly kitchen worksurface - but really whatever I can find second hand

So it'a a piece of string question really

Ok

I'm now doing the job

Worksurface is 3.2m ideally I'd like a 'table' so not fixed to the wall

I thought a 400 unit on either end then I have 6 chrome legs

So 400 unit 600 to leg, 600 to leg, 600 to leg, 600 to 400 unit - would that work?

To the legs need to be even or would they be better sort of diagonally or would I need central legs too?

Or possibly an extra 400 unit? Centrally?

I think you are saying the unsupported length will be 600mm

that's quite normal, bridging over slide-in appliances.

A metre would be too much.

You can fit a batten under the unsupported front edge. For beam depth, it is the height of the batten that gives strength, much more than its thickness.

I wasn't going to fit it onto a wall - so it would be freestanding?

The gaps would include the width of the legs

As a freestanding table, those legs are not very good. Imagine if it was heavy and got nudged sideways, e.g. someone fell against it. The legs would tend to bend sideaways and wrench out the screws.

Fair point May need a rethink

What sort of legs would I need?

wooden tables usually have a subframe where all the legs are rigidly connected to each other in a rectangle. I don't know how to do that in your case. There may be a tubular metal structure you could use. Perhaps browse some conference room tables.

If you could fix it rigidly to the wall, that would stop the top moving back or sideways.

Maybe having it freestanding isn't possible then