Hey, I ended up staying in Lancaster over summer, and I'd say it's only a good idea if you have friends who are also staying.
I found a place for £25 per week plus bills (cheap summer rent, normally £50/week in term time) and I was living in a flat with 2 other people. They kept to themselves and didn't really want to hang out (or even talk for that matter....) and kitchen was gross because the landlord didn't make them clean up before new people moved in (they'd lived there in the term as well as they were international students). The place would have been OK if it had been cleaned though, and my room was fine.
I had a job in Preston (I have a car) and I drove to work a few times a week, which was fine for me.
However, as I didn't know anyone, I ended up going to my friends flat in Preston a lot and in the last 3 weeks of the summer I moved home anyway because I was pretty bored.
If you want the details of the place, let me know, it's £30 a week this summer, although I'd only reccommend it if you're moving into a flat with people who are moving in for summer (so the landlord has to make the previous occupants clean up, lol!)
This summer, I'm going home, as I want to save money, but am going to be in Lancaster quite a bit anyway as my boyfriend lives in a house which he has all year anyway.
Hope that helps!
Nic
[edited to add: I paid my deposit (£150) a week or two after Easter, and my friend was going to move in but decided too late (4 weeks after me) and there were no rooms left.]