I got one from b&q lasted year and its fine for 99 quid. Think it's made by a company called rage.
Rage 3-S Evolution are worth their weight in gold. £99 Screwfix.
The Thin Multi purpose Blades are Amazing, and they are only £20 to replace, but they last ages.
(Dependant on how much Metal you cut through)
I have a £700 Bosch 305mm Mitre Saw that I used for work, but I bought the Rage for smaller jobs.
Since I've had the Rage saw, I've only ever used the Bosch for things like 4x4" decking posts etc. The Rage is So handy and portable.
The Rage is awesome for Skirting and Door Archi's etc. Even with its little Blade it will cut 200mm on a 45'.
You will struggle with wet joists if they are over 6x2" but tbh the Rage 3-S isn't really the tool for that sort of work.
If you get the Rage 3-S you WILL love it.
http://www.evolutionpowertools.com/uk/build/rage3s.html