[note: after posting, I now see you're in the UK. I'll try to see what brand names some good equipment might be sold under over there, if you can't fix your current filter or find the brands people in the US mention.)
Ok, I'm certainly not a Betta expert, but freshwater aquarium equipment, now that I'm an expert in! (or so I like to flatter myself) Yep, being a tech-geek spills over into my aquarium hobby.
Anyway, inside the pump head (the black thing on top,) there's an impeller. Open up the pump head (the instructions should say how, since you have to clean the impeller occasionally,) and make sure it's seated properly down in the well. Also make sure there's no gravel or anything else in the impeller well.
Here's what an impeller looks like: http://www.petmountain.com/category/...impellers.html
If you turn the filter off every night, chances are that you'll fail to grow a proper biological filter (the bacteria that eat ammonia and nitrite,) and your water will become polluted. If you can't make this filter work, I recommend you get a better, well-known brand. The Duetto filters are nice but perhaps too large for 2.5 gallons? I also looove my Red Sea Nano external filter. I have it on a 2 gal. hex. I keep it turned almost all the way down. It's dead silent. I read Petco stocks them for $12.
Sorry if I repeated any "obvious" info you already know, but I like to cover all the bases, just in case. I never mean to talk down to people like they're idiots, but it sounds like it sometimes.