[Flump]

how do you know when it is ready to eat?

[arugula]

From the BBC gardening web pages:

"In its first year, plants need to put all their energy into making growth, so remove any flowerheads as they form.

In the second year, allow the edible heads to develop for harvesting in summer. Pick the terminal bud (the one at the top) first, when it's large and swollen, but before the scales have started to open - cut off with a few centimetres of stem attached. Pick the side buds when they have reached a decent size."

I hope to have this problem soon too, as mine are in their second year.

[Flump]

thanks for that Mine are in the third year now,but didn't get to them quickly enough last year!only had a couple.The earwigs like them too 

[mumofstig]

I've got one top bud I'm keeping a beady eye on, and it seems more like a prurple rather than a Green Globe.

I know grown from seed they're variable, but I never expected that  

You have to watch out for blackfly, they're awful if they get in the scales *Bleugh*

[arugula]

Is it reasonably easy, or not, to tell by looking how big the heart is whilst the head is intact on the plant?

You obviously want to leave it long enough for the heart to be a decent size, but not too long that it goes over or something gets it.

[Flump]

I don't find it easy!

[mumofstig]

It's like broccoli one minute it's green the next it's flowering 

this site has pics which may help (scroll down to Harvest )
http://pubs.ext.vt.edu/438/438-108/438-108.html

[Flump]

Do you know the best way to eat it Mum?

[mumofstig]

I like them cooked whole and the leaves dipped in garlic mayo or a lemony vinaigrette, or the bases in a sauce for pasta

Preparation explained and a few different sauces 
here

The prep, if you just want to use the hearts is shown in more detail
here

[Flump]

oh ta! hope I get chance to go up there and pick some if the rain stops!