You might get lucky and pick up a second hand 1.1l in the classifieds for 50ish, no doubt going to cost you abit more, you could probably pick up 2 2nd hand steel 0.8l bottles or buy a new 0.8l bottle for around £30-40 in most shops.
your going to need 20 posts before you can use the classifieds though.
I echo what @BDswarveP said. You're going to have to be very lucky to pick up a 1.1l for £50, a CF one anyway. Could quite easily get yourself a steelie. They easily available for below £40 brand new.
I'd save up and for a 4500 psi cylinder, the others are right, you'll be lucky to pick up one for that money unless it's out of date, and even then you'll still need to have it tested adding more onto the cost.