Has anyone used a power inverter - 300watt sort of size for any length of time ?
We have a 2ft 18watt strip lamp and were looking to charge the mobile phone possiblely lap top as well - would this be a good idea or would it kill the car battery quite quickly ?
car batteries are not designed for sustained use like a leisure battery, a car battery delivers high levels of current for short period of time e.g to turn your starter motor on your car, leisure batteries differ in that they provide a sustain amount of power over a long period of time.
in your case the inverter will probably flatten your battery in an hour or two of use if you use all the item you quote especially the laptop !, I think you will be OK with the lights & mobile.
300w will do the trick for your requirements. If you want to do more exciting things then you will need more battery power. This will involve fitting a "leisure" type battery. If you only use to charge the laptop when you are driving you shouldn't have a problem.
The Aldi one is only 150w, but I've even use a 150w invertor to run a 100w PA system!
The things I like about the Aldi over my old 150w are the shape - it sits in a drink holder, and the USB port. A lot of things now use USB, the phone makers have agreed to use USB as a new power supply standard.