What size chainring have you got on the front just now? If its a 22 tooth then all you will need is a freewheel. 22-18 is pretty much the same as 18-15. I would recommend just getting a new freewheel and putting it on the rear.
Yeh, Cause I never new Tarty were good before... To be honest I think everybody already knows that Tarty are the best. They own. T-UK on the other hand.
I had an Echo Freehub, the ratchets are only glued in for the deng freehubs so the come loose and you pedal but nothing happens. This happened to mine so I got a TR Hub and SL Freewheel replacement which is really good.