whats the depth of the screed mate?
does it need to be flexible (latex mix)
if its just a level of upto 5mm, then its really easy. mix, pour and go...dont over trowel. mix in a gorilla tub , pull the handles together to create a spout and pour from one corner to the other, very lighter glide your trowel over.
if in future you level larger areas, then go and buy a 22in or 33in notched levelling rake from
www.refina.co.uk excellent and less hassle...
its only a 1 hour job, so i would charge £50 plus mats and do it either on the way to a job or on the way home, that way you'll get your normal day rate and £50 ontop. should put you in the 200 a day bracket then mate ;0)