Fitting a new water main is a fairly easy project to undertake:

First you must dig your trench750 mm( 2`6") deep,which will need to be inspected by your local water authority so dont fill it in whenyou finish.You will need to dig all the way under your garden wall,hedge or border of your property so that the water board can pull the pipe to the main without digging inside your property.At this end leave around a metre of pipe for them to work with.
On entry to the building you will need to fit your plastic pipe inside 4" drainage fittings where they may be concreted or if you have to turn a corner to ensure the pipe will not kink or turn to quickly.All pipes inside the building will then need to be lagged with pipe lagging to make sure they cant freeze during the winter months.At the end of your pipe fit a plastic to copper fitting.Usually this will be a plastic to copper stop tap but this will obviously depend on the purpose of your new mains.Then you will need to call the local water company for them to come & inspect that the work is up to the standards they require.They will then book the job in joined up to the mains.Normally all in all this is quite a fast process & from start to finish will take a couple of weeks.