Get a good rake and rake out the moss. It should take you a whole day. Get right down to the soil. Then come along with a fork and dig in lifting the said area up about 3 inch,s airating the soil underneath. 1. The raking of the moss gives the grass a better chance of growing 2. The lifting of the sod "Mossy Area", airates the soil, drys it out, takes the heavy wet out of it, Moss will only grow in a damp wet area. Make sure you do the whole lawn. Worst comes to worst. Dig a deep trench down one side of the garden, At least a metre, fill it with small stone or waste stone. Then fill back in. Make sure you dig it on the down slope of the garden. This way the whole lawn area will have a natural drain.