To clarify a few things:
No, there's no hidden catch. Until a few years ago it was mandatory to join a union which cost like £80 a year, but nowadays there's no mandatory fees. Obviously you still have to buy books etc if you don't like libraries.
Regarding living expenses:
Most Swedish people that go to England tells me that they pay twice as much in rent in the UK for half the standard. We jokingly call England a "third world country", because of things like carpet floors, old-fashioned vacuum cleaners, bad isolation, bad heating, different taps for hot and cold water etc... Friends in Scotland have even told me that they wear winter clothes inside - while I can walk around in underwear in my dorm room without feeling the least bit cold.
In Uppsala and Lund, you pay £300 for a room on average. In Umeå, £200. In Stockholm, a lot more - at least £400.
All masters programmes in Sweden are taught in English. About 50% of masters students are foreign, but the number is higher in the sciences.
PhD studies pays about £2000 a month as it is considered a job/employment in Sweden. However, you need a masters to be eligible (we don't do the whole 1+3 thing for example). However, certain masters programmes, such as biology at Uppsala, have a 35% rate of students who start PhD studies upon finishing.
A use-as-much-as-you-want transport card for one month is about £30 in almost all cities, except in Stockholm where it's about £100.
Alcohol tends to be more expensive, since Sweden has high alcohol taxes. A drink with 4 cl in Umeå/Uppsala/Lund at a student bar is about £5, and about £8 in Stockholm.
Regarding food, you can usually get a lunch out for about £5. £10 in Stockholm. I never spend more than maybe £150-200 a month on food.
Swedish students live on ~£800 a month and that's a decent budget. Won't do in Stockholm, but you'll be fine everywhere else.