Winter lettuce and beans

Lettuce will crop over winter in a gh, but beans won't

Most people on here will start things off in small pots Polycarb, and pot them on as necessary. With lettuce though, I'm pretty sure you could sprinkle your seed over the surface of a larger pot and just leave it to grow, picking some as baby leaves and leaving a few to grow to full size if you want to.

Beans - you're a bit late with these for this year, unless you have some which specifically say you can plant until August? You could, however, sow some varieties of peas which would overwinter in your greenhouse and give you an early-ish harvest next year? Or you can grow them for pea shoots I believe - I've not done this yet, but I'm sure someone will tell you how big they need to be before you pick them etc. Shoots would give a winter crop I guess.

There are other things you can still plant now if you want to, though they won't all crop this year... :

spring onions
spring cabbage
carrots (check variety)
kohl rabi
autumn onion sets
chard
spinach (I'm using perpetual / winter spinach which was fab in my tunnel last year)
lambs lettuce
pak choi
rocket
mizuna and other oriental salads and greens

So no need to leave it empty!

Can't you start broad beans in October and leave them to overwinter?

^^I do, but I grow them outside, you'll need someone with a tunnel to tell you if they grow inside

You can grow them to overwinter Spunky yes - and anything you can grow outside, you can also grow in a polytunnel. Just don't plant too early or they'll get ahead of themselves!