Installing a cog on hub

How the hell do I take the freewheel off? More to the point is it worth me just going down to BLB and giving them a fiver to do it right?

Also will going from a 16t to a 18t kill me?

http://www.parktool.com/repair/readhowto.asp?id=48
See about half way down for pictures of the plethora of tools that may be required, and a link how to destructively remove a freewheel using a pin spanner and a vice in case no other tools are suitable.

Does the hub have a reverse thread?

i.e. looking carefully at where the freewheel attaches to the hub, might the threading be in two parts like in the OP's second photo, with your freewheel engaged only with the wider, right-handed part of the thread, leaving the threads at the end of the freewheel you can see hovering over thinner left-hand threads? If not, your hub probably doesn't have the section of thinner reverse thread required to fit a lock ring, and you need to think again about this project.