O_O
6 bed hostel rooms aren't very safe if you have lots of stuff! I was in one in NYC and my watch fell off during the night, I heard it clunk but was too tired to get it, when I woke up (early) in the morning it was gone.
I hope you've thought about some options of where you want to keep stuff, even with lockers etc hostels can be dens of thieves.
Maybe a short term rental from spareroom.co.uk would be better?
Yes.
I initially stayed in hostels back when I did BUNAC. Hostels can be full of people with no moral compunction against stealing other people's things. Sadly, I experienced this firsthand.
A few tips:
Put a lock on everything. Use combination locks instead of key locks. One, you won't have keys to lose, and two, key locks are easily picked. Combination locks can be cracked, too, but it takes a bit more time.
The hostel may advertise "lockers available," but this can mean different things. One hostel I stayed in said this, but what that meant was one lockable cage per bunk bed unit. If your bunkmate got there first and took it, you were out of luck.
Try to avoid using a standard-looking laptop case for your laptop. It advertises, "hey, I have a laptop." Many backpacks have laptop areas, or you can purchase laptop bags that are designed like backpacks. Lock it.
I slept with my handbag under my pillow. I kept my iPod and camera in my handbag. Take it to the shower with you. You have no idea who your roommates are or what kind of people they are. Thieves are opportunistic.
Take your laptop with you when you leave the room. If you don't want to take it out with you every day, leave it in the hotel luggage room. Only the hotel staff has keys and you can only retrieve your bags with a claim check. Not all hostels have luggage rooms. Check before booking to see if yours does.