Just collected bike from dealers totally forgot to test even though most probably would not of with suit on. Anyway only have one brake on bike which is not biting at all (XT Hydraulic), please someone advise what I need to do. Thanks in advance

I'd guess they haven't bled it properly, most of the modern Shimano stuff now seems to require it before use, either that or they may need aligning and bedding in properly.

return it to the dealers to be sorted?

go to shimano or park tools website, easy with shimano bleed kit

Bleeding is not always the answer. Unless you are used to working on brakes take it back. You paid extra to buy it in a dealer, may as well take advantage of being able to return it easily.

Deffo take it back.

Going back first thing what a waste of time!