AIUI 18v drills have more 'oomph'. Is this true in all cases? A 1 hour charger you can also leave on to keep the battery topped up would be good. A spare battery would be nice also :-)
There are some remarkably cheap 18v drills e.g. http://www.argos.co.uk/static/Product/partNumber/7112812/c_1/1 |category_root|Garden+and+DIY|14418702/c_2/3|15701048|Power+tools|14418763/c_3/4|cat_14418763|Cordless+drills+and+screwdrivers|14418778.htm?_$ja=tsid:11527|cc:|prd:7112812|cat:garden+%26+diy+%3E+power+tools+%3E+cordless+drills+and+screwdrivers but this doesn't seem to have a second battery and takes 3-5 hours to charge. [My B&Ds take 3 hours to charge with dire warnings about leaving them charging for too long, and needing to run them down each time to avoid loss of performance.]
An alternative would be a power (240v) drill/screwdriver which had a ratchet torque setting - like the cordless drills which go 'bradadadadaada' like an impact driver when you hit the torque setting. My ancient B&D Quattro has a torque stall setting but this isn't as effective or controllable and the drill tends to stop short or slam the screws home. It also doesn't always work - I managed to smoke the drill when fitting M10 screws although it seems to have survived so far.
Given that it is a spending frenzy Bank Holiday weekend I thought I might look for a new driver before I put in all the screw bolts on my metal roof. These need to be just 'so tight' to avoid over compressing the rubber seal.
Do I have to go up to £90-£100 to get a decent drill/driver?