Another vote for Montane.
I own a pair of Terra Pack pants and use them for the summer months. My med/reg length pair weighs in at 210g - packs down into a small (supplied) storage bag, has three press studs at the ankle to adjust fitting, and comes with its own belt. My only gripe would be that the pockets are a little on the small side. A quick online search finds them for sale at £41.99 + £4.99 postage.
I can also recommend the Simond Alpinism lightweight trousers from Decathlon. They keep the rain out well and are super stretchy when worn. They have two front, zipped pockets and come with a decent belt. I'd recommend trying them on in the store though as size info was a little confusing (I'm 5'8, 145lbs with a 30" waist and ended up buying size EU:42 / US: small/med. They cost £34.99