Circular saw for cutting flat roof materials

[oly]

Can anyone suggest a decent circular saw to make my flat roof please? It won't get a lot of use after this, unless I suddenly take up building as a profession.

Timbers are 47 x 195 and OSB 18mm and mostly 90 degree cuts.

The saw I currently have was VERY cheap and I'd like a safer one, perhaps with immediate blade stop like a chainsaw (if there is such a thing). My budget is £50-£100-ish though if I have to pay more I guess I will.

Cheers for any advice...

 

[jeds]

For good value DIY stuff I don't think you can beat the Macallister range from B&Q. For about a oner you could buy the skill saw and a cross cut saw.