grass

Dig it up spit at a time and place it to one side with the grass facing down. Theoretically it should die off and leave you with a mound of soil. DON'T use weedkiller unless you are planning a long time before you get any crops in the ground.

Digging in would work better when done in autumn and the grass has all winter to rot down. It would be easier now if you were to strim or mow the grass first and compost it seperately

Depends how organic you want to be -  you will will get loads of people advocating use of glyphosate. It was claimed to be safe and leave no residue in the soil but many are no doubting that and it has been banned in some countries. BEWARE

weed killer or not thats the question
maybe i will install the outside tap 1st and have a think about it

A word of warning: if you're going to cover it over, make sure whatever you cover it with really doesn't let light in.  I had part of my plot covered with cheap weedproof fabric from October onwards and when I took it off last week I found I'd grown a great crop of couch grass!  The fabric was good enough to suppress annual weeds, but for things like couch grass which store energy in their roots it let just enough light through to let them keep growing away.  It did do a good job of keeping the soil moist and warm, so at least it's easier to dig the roots out than if I'd left it uncovered, but if I want to do it again I'm going to buy some expensive stuff!