thinking of fitting an outside tap foa a hosepiep was wondering if the self tapping kits like this Garden Tap Kit - Screwfix.com, Where the Trade Buys (http://www.screwfix.com/prods/10101/Plumbing/Water-Pipe-and-Fittings/Garden-Tap-Kit)
are ok

worried about what happens if they start to leak or would I be better using normal brass/copper fittings for everything?

Depends what you want - they're OK for a tenner if you want to fill up a watering can or something like that, but are narrow bore, so don't flow as well as 15mm tube if you're running a hoze or a pressure washer.

Also make sure you tap into a mains pressure cold pipe and not a pipe supplied from the cold water tank or you'll have even less pressure.

^^^^^^^^
What he said;)

Alcazar