If it's never been serviced before then that's why it skips. They come with grase in them to bed them in, once that's done, you need to service it. Take the lockring off, I think it's reverse thread on Tensiles? Then once that's off, take out the bearings, and give them a clean with a bit of kitchen roll. Grab a can of GT85, and spray it all inside the freewheel, but not on the bearings! Once it's all soaked, get the cloth and give it all a good wipe clean. Put all the bearings back in, and drip some wet lubricant in there, Finish Line oil is a good'un, as is the Halfords Bikehut one. Get some of that inside to keep it all lubricated, and you'll be good to go. You don't want to use grase as it's too thick and will cause it to skip again.