Question
We recently moved into a house with a garden riddled in bramble/blackberry bushes and nettles after stripping them back and digging up as many roots as a can we can see that the grass is full of weeds and some of the weaving roots from the brambles have gone underneath, we presume we are going to have take up the lawn and replace it, what is the best way of doing this and how can I stop new weeds from ruining a new lawn?

Answer
Given the disastrous state you inherited with those brambles/blackberries/nettles, not to mention the millions - MILLIONS - of weed seeds waiting to hatch around your house, I would say you need to use extreme measures to save your real estate.

I want to warn you before we start, however, that chemicals for this purpose are out of the question.  I say that because many people these days reach for the most potent toxic waste they can find to obliterate all signs of life in a mess like yours.  Chemicals will wipe out soil microbes on a level that will take years to repair.  That would be a real tragedy given the robust populations that must be growing there are a result of neglect.  They enrich the soil and are directly responsible for soil fertility on a level that most of us can only dream about.  Support your local soil microbes.

There are 2 ways to handle your situation.  You can use either, or you can even do both of them, depending on your own assessment of what is needed.

Blackberry bushes and the other shrubs you mention will return with a vengeance if you simply try to cut them back and remove them the easy way.   Cover the area with a disposable carpet or with thick black polythene sheets that are designed to stop weeds, and weigh down with stones.
Wild Blackberry shrubs will develop out of any root fragment left in the soil.  This is in fact considered one of the worst noxious weeds by the Australian Government.

Given those facts, you should understand that this is not going to be an easy task.  Several seasons of diligent attack will be needed for you can count on long term results.

The black landscape fabric used to cover soil to prevent weeds from growing would be good to cover mowed-down shrubs.  In this case, the fabric (or alternately you can use black plastic) would be removed regularly - as much as you can bother - so that the shrubs can be re-mowed or re-cut.  The longer you do this, the better your results.

Solarization is a backup and needed to get rid of all the miscellaneous weeds that have landed there and gone to seed with the previous residents.  You use clear plastic for this and make sure the soil is moist to trigger germination in the millions of weed seeds waiting there.

In the late fall, if the soil looks ready, it will be time to put down sod.